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.

* Added SMTP section to --request
* Expanded --mail-rcpt to describe the usage when using the verify and
  expand commands.

Although this option should have already been set, the SMTP module can
now download information from and send instructional commands to, an
SMTP server, requiring the option to be set in order to perform a mail
transfer.

Incorrectly processed multiline server greeting responses as "wanted"
continue responses in smtp_endofresp(), from commit f16c0de4,
which in turn broke the SMTP server detection in the test suite,
because the EHLO command would not be sent.

warning: unused variable 'smtpc'

Added a loop to smtp_statemach_act() in which Curl_pp_readresp() is
called until the cache is drained. Without this multiple responses
received in a single packet could result in a hang or delay.

Similar to the processing of untagged CAPABILITY responses in IMAP moved
the processing of multiline EHLO responses to smtp_state_ehlo_resp() and
introduced an internal response code of one to differentiate a multiline
continuation from the end of command. This also allows for the separate
processing of multiline responses from commands such as VRFY and EXPN.

singleipconnect() did not return the open socket descriptor on some
errors, thereby sometimes causing a socket leak. This patch ensures
the socket is always returned.

The ipv6 auto-detect test in configure returns a false negative when
CFLAGS contains -Werror=implicit-function-declaration. (I have been
using this flag to detect code issues that would result in SEGVs on
x86_64-cygwin.)

Patch-by: Yaakov Selkowitz
Bug: http://curl.haxx.se/bug/view.cgi?id=1304