Prior to this change we called Curl_ssl_getsessionid and
Curl_ssl_addsessionid regardless of whether session ID reusing was
enabled. According to comments that is in case session ID reuse was
disabled but then later enabled.

The old way was not intuitive and probably not something users expected.
When a user disables session ID caching I'd guess they don't expect the
session ID to be cached anyway in case the caching is later enabled.

Calling QueryContextAttributes with SECPKG_ATTR_APPLICATION_PROTOCOL
fails on Windows < 8.1 so we need to disable ALPN on these OS versions.

Inspiration provide by: Daniel Seither

Closes #848
Fixes #840

Closes #845

Sessionid cache management is inseparable from managing individual
session lifetimes. E.g. for reference-counted sessions (like those in
SChannel and OpenSSL engines) every session addition and removal
should be accompanied with refcount increment and decrement
respectively. Failing to do so synchronously leads to a race condition
that causes symptoms like use-after-free and memory corruption.
This commit:
 - makes existing session cache locking explicit, thus allowing
   individual engines to manage lock's scope.
 - fixes OpenSSL and SChannel engines by putting refcount management
   inside this lock's scope in relevant places.
 - adds these explicit locking calls to other engines that use
   sessionid cache to accommodate for this change. Note, however,
   that it is unknown whether any of these engines could also have
   this race.

Bug: https://github.com/curl/curl/issues/815
Fixes #815
Closes #847

Closes #822

For the Windows XP toolset of Visual C++ 2013/2015, the old Windows SDK
7.1 is used. In this case, _USING_V110_SDK71_ is defined.

Closes #812

Only protocols that actually have a protocol registered for ALPN and NPN
should try to get that negotiated in the TLS handshake. That is only
HTTPS (well, http/1.1 and http/2) right now. Previously ALPN and NPN
would wrongly be used in all handshakes if libcurl was built with it
enabled.

Reported-by: Jay Satiro

Fixes #789

warning C4244: '=': conversion from 'int' to 'unsigned short', possible
                    loss of data

Add ALPN support for schannel. This allows cURL to negotiate
HTTP/2.0 connections when built with schannel.

Closes #724

Regression from commit 7a8e861a as highlighted in the msys autobuilds.

schannel.c:1125:5: warning: missing initializer for field 'dwMinorVersion'
                   of 'OSVERSIONINFOEX' [-Wmissing-field-initializers

New tool option --ssl-no-revoke.
New value CURLSSLOPT_NO_REVOKE for CURLOPT_SSL_OPTIONS.

Currently this option applies only to WinSSL where we have automatic
certificate revocation checking by default. According to the
ssl-compared chart there are other backends that have automatic checking
(NSS, wolfSSL and DarwinSSL) so we could possibly accommodate them at
some later point.

Bug: https://github.com/bagder/curl/issues/264


Reported-by: zenden2k <zenden2k@gmail.com>

This commit is several drafts squashed together. The changes from each
draft are noted below. If any changes are similar and possibly
contradictory the change in the latest draft takes precedence.

Bug: https://github.com/bagder/curl/issues/244
Reported-by: Chris Araman

%%
%% Draft 1
%%
- return 0 if len == 0. that will have to be documented.
- continue on and process the caches regardless of raw recv
- if decrypted data will be returned then set the error code to CURLE_OK
and return its count
- if decrypted data will not be returned and the connection has closed
(eg nread == 0) then return 0 and CURLE_OK
- if decrypted data will not be returned and the connection *hasn't*
closed then set the error code to CURLE_AGAIN --only if an error code
isn't already set-- and return -1
- narrow the Win2k workaround to only Win2k

%%
%% Draft 2
%%
- Trying out a change in flow to handle corner cases.

%%
%% Draft 3
%%
- Back out the lazier decryption change made in draft2.

%%
%% Draft 4
%%
- Some formatting and branching changes
- Decrypt all encrypted cached data when len == 0
- Save connection closed state
- Change special Win2k check to use connection closed state

%%
%% Draft 5
%%
- Default to CURLE_AGAIN in cleanup if an error code wasn't set and the
connection isn't closed.

%%
%% Draft 6
%%
- Save the last error only if it is an unrecoverable error.

Prior to this I saved the last error state in all cases; unfortunately
the logic to cover that in all cases would lead to some muddle and I'm
concerned that could then lead to a bug in the future so I've replaced
it by only recording an unrecoverable error and that state will persist.

- Do not recurse on renegotiation.

Instead we'll continue on to process any trailing encrypted data
received during the renegotiation only.

- Move the err checks in cleanup after the check for decrypted data.

In either case decrypted data is always returned but I think it's easier
to understand when those err checks come after the decrypted data check.

%%
%% Draft 7
%%
- Regardless of len value go directly to cleanup if there is an
unrecoverable error or a close_notify was already received. Prior to
this change we only acknowledged those two states if len != 0.

- Fix a bug in connection closed behavior: Set the error state in the
cleanup, because we don't know for sure it's an error until that time.

- (Related to above) In the case the connection is closed go "greedy"
with the decryption to make sure all remaining encrypted data has been
decrypted even if it is not needed at that time by the caller. This is
necessary because we can only tell if the connection closed gracefully
(close_notify) once all encrypted data has been decrypted.

- Do not renegotiate when an unrecoverable error is pending.

%%
%% Draft 8
%%
- Don't show 'server closed the connection' info message twice.

- Show an info message if server closed abruptly (missing close_notify).

Some servers will request a client certificate, but not require one.
This change allows libcurl to connect to such servers when using
schannel as its ssl/tls backend. When a server requests a client
certificate, libcurl will now continue the handshake without one,
rather than terminating the handshake. The server can then decide
if that is acceptable or not. Prior to this change, libcurl would
terminate the handshake, reporting a SEC_I_INCOMPLETE_CREDENTIALS
error.

https://github.com/bagder/curl/issues/244

Commit 145c263a changed the behavior when Curl_read_plain returns
CURLE_AGAIN. We now handle CURLE_AGAIN and SEC_I_CONTEXT_EXPIRED
correctly.

Commit: https://github.com/bagder/curl/commit/926cb9f
Reported-by: Ray Satiro

Bug born in changes made several days ago 9a91e805.

Bug: http://curl.haxx.se/mail/lib-2015-04/0199.html
Reported-by: Brian Chrisman

Reported-by: Brian Chrisman

white space changes only

No need to use _MPRINTF_REPLACE internally.

Removed the curl_ prefix from the schannel source files as discussed
with Marc and Daniel at FOSDEM.

If the session is still used by active SSL/TLS connections, it
cannot be closed yet. Thus we mark the session as not being cached
any longer so that the reference counting mechanism in
Curl_schannel_shutdown is used to close and free the session.

Reported-by: Jean-Francois Durand

The return type of this function is a boolean value, and even uses a
bool internally, so use bool in the function declaration as well as
the variables that store the return value, to avoid any confusion.

- do not grow memory by doubling its size
- do not leak previously allocated memory if reallocation fails
- replace while-loop with a single check to make sure
  that the requested amount of data fits into the buffer

Bug: http://curl.haxx.se/bug/view.cgi?id=1450
Reported-by: Warren Menzer

Bug: http://curl.haxx.se/bug/view.cgi?id=1462
Reported-by: Tae Hyoung Ahn