OPENSSL_load_builtin_modules does not exist in BoringSSL. Regression
from cae43a10

The symbol SSL3_MT_NEWSESSION_TICKET appears to have been introduced at
around openssl 0.9.8f, and the use of it in lib/vtls/openssl.c breaks
builds with older openssls (certainly with 0.9.8b, which is the latest
older version I have to try with).

** WORK-AROUND **

The introduced non-blocking general behaviour for Curl_proxyCONNECT()
didn't work for the data connection establishment unless it was very
fast. The newly introduced function argument makes it operate in a more
blocking manner, more like it used to work in the past. This blocking
approach is only used when the FTP data connecting through HTTP proxy.

Blocking like this is bad. A better fix would make it work more
asynchronously.

Bug: https://github.com/bagder/curl/issues/278

CVE-2015-3236

This partially reverts commit curl-7_39_0-237-g87c4abb

Reported-by: Tomas Tomecek, Kamil Dudka
Bug: http://curl.haxx.se/docs/adv_20150617A.html

CVE-2015-3237

Detected by Coverity. CID 1299430.

Bug: http://curl.haxx.se/docs/adv_20150617B.html

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).

s/curret/current/

* use SSL/TLS where available
* follow permanent redirects

"At condition p_request, the value of p_request cannot be NULL."

Coverity CID 1306668.

... by removing the "do {} while (0)" block.

Coverity CID 1306669

use \fI-style instead of .BR for references

... to simplify checking when PUT _or_ POST have completed.

Reported-by: Frank Meier
Bug: http://curl.haxx.se/mail/lib-2015-06/0019.html

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.

and a conversion to markdown. Removed the lib/README.* files. The idea
being to move toward having INTERNALS as the one and only "book" of
internals documentation.

Added a TOC to top of the document.

Although OpenSSL 1.1.0+ deprecated SSLv23_client_method in favor of
TLS_client_method LibreSSL and BoringSSL didn't and still use
SSLv23_client_method.

Bug: https://github.com/bagder/curl/commit/49a6642#commitcomment-11578009


Reported-by:  <asavah@users.noreply.github.com>