Added a simple example to show how one can use CURLINFO_TLS_SESSION for
obtaining extensive TLS certificate information.

Systems that define SIGPIPE_VARIABLE as a noop would not compile as
restore_pipe was defined afterwards.

This fixes a NULL dereference in the case where the client asks for
CURLINFO_TLS_SESSION data after the (TLS) session has already been
destroyed (i.e. curl_easy_perform has already completed for this
handle). Instead of crashing, we now return a CURLSSLBACKEND_NONE
error.

This is an extension to the fix in 7d80ed64

. We may
call Curl_disconnect() while cleaning up the multi handle,
which could lead to openssl sending packets, which could get
a SIGPIPE.

Signed-off-by: Jeff King <peff@peff.net>

Commit 7d80ed64

 introduced some helpers to handle
sigpipe in easy.c. However, that fix was incomplete, and we
need to add more callers in other files. The first step is
making the helpers globally accessible.

Since the functions are small and should generally end up
inlined anyway, we simply define them in the header as
static functions.

Signed-off-by: Jeff King <peff@peff.net>

This fixes a rare Happy Eyeballs bug where if the first IP family runs
out of addresses before the second-family-timer fires, and the second
IP family's first connect fails immediately, no further IPs of the
second family are attempted.

When adding entries to the DNS cache with CURLOPT_RESOLVE, they are
marked 'inuse' forever to prevent them from ever being removed in normal
operations. Still, the code that pruned out-of-date DNS entries didn't
care for the 'inuse' struct field and pruned it anyway!

Reported-by: Romulo A. Ceccon
Bug: http://curl.haxx.se/bug/view.cgi?id=1303

A failure during authentication, which is performed as part of the
CONNECT phrase (for IMAP, POP3 and SMTP) is considered by the multi-
interface as being closed prematurely (aka a dead connection). As such
these protocols cannot issue the relevant QUIT or LOGOUT command.

Temporarily fixed the test cases until we can fix this properly.

Removed unnecessary SMTP_STOP state changes on failure.

Removed hard return on failure in smtp_state_data_resp().

Added new API for returning a SSL backend type and pointer, in order to
allow access to the TLS internals, that may then be used to obtain X509
certificate information for example.

Commands such as NOOP, RSET and HELP would be sent with a space at the
end of the command, for example: "NOOP ".

Otherwise a NOOP operation would be performed which a) only returns a
single line response and not a multiline response where -I needs to be
used, and b) provides an inconsistent user experience compared to that
of the POP3 and IMAP protocols.

The buffer allocated by smtp_parse_custom_request() was not freed.

singleipconnect() could return the file descriptor of an open socket
even though the function returned a CURLE_COULDNT_CONNECT error code
from commit ed1662c3 and 02fbc26d.

This could cause tests 19, 704 and 1233 to fail on FreeBSD, AIX and
Solaris.

The error code should not be sent as data as it isn't passed onto the
client as body data, so cannot be compared in the test suite against
expected data.

* Added information about the verify and expand commands to
  CURLOPT_MAIL_RCPT.
* Reworked CURLOPT_CUSTOMREQUEST section, adding information about IMAP
  and SMTP custom commands.