Not all SASL enabled POP3 servers support the AUTH command on its own
when trying to detect the supported mechanisms. As such changed the
mechanism detection to use the CAPA command instead.

- For all *FUNCTION options, they now all show the complete prototype in
  the description. Previously some of them would just refer to a
  typedef'ed function pointer in the curl.h header.

- I made the phrasing of that "Pass a pointer to a function that matches
  the following prototype" the same for all *FUNCTION option descriptions.

- I removed some uses of 'should'. I think I sometimes over-use this
  word as in many places I actually mean MUST or otherwise more specific
  and not-so-optional synonyms.

Currently 1348 to 1354 succeed but a write failure is logged in traceNNNN.

Currently 1355 fails, so disabled for now.

Reworked variable names in Curl_sasl_create_cram_md5_message() to match
those in Curl_sasl_create_digest_md5_message() as they are more
appropriate.

Moved the digest-md5 message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.

Correction of comments and variable names.

Added some comments and removed an unreferenced variable.

Added USE_NTLM condition compilation around the NTLM functions called
from pop3_statemach_act() introduced in commit 69f7156a.

Moved the cram-md5 message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.

Because pop3_endofresp() is called for each line of data yet is not
passed the line and line length, so we have to use the data pointed to
by pp->linestart_resp which contains the whole packet, the mechanisms
were being detected in one call yet the function would be called for
each line of data.

Using curl with verbose mode enabled would show that one line of data
would be received in response to the AUTH command, before the AUTH
<mechanism> command was sent to the server and then the next few lines
of the original AUTH command would be displayed before the response from
the AUTH <mechanism> command. This would then cause problems when
parsing the CRAM-MD5 challenge data as extra data was contained in the
buffer.

Changed the parsing so that each line is checked for the mechanisms
and the function returns FALSE until the whole of the AUTH response has
been processed.

Due to new features

Also bumped the contributor number and next release is to become 7.27.0

Added new sections 11. IMAP and 12. LDAP to document adding SASL based
authentication.

Renumbered current sections 11 to 17 as 13 to 19.

Additionally added 19.10 Add CURLOPT_MAIL_CLIENT option.

Applied a fix to avoid warnings on systems where Curl_ntlm_sspi_cleanup()
is just a nop.

Previously it wasn't possible to connect to POP3 and not specify the
user name as a CURLE_ACCESS_DENIED error would be returned. This error
occurred because USER would be sent to the server with a blank user name
if no mailbox user was specified as the server would reply with -ERR.

This wasn't a problem prior to the 7.26.0 release but with the
introduction of custom commands the user and/or application developer
might want to issue a CAPA command without having to log in as a
specific mailbox user.

Additionally this fix won't send the newly introduced AUTH command if no
user name is specified.

Corrected lines exceeding 78 characters.

Repositioned some comments and added extra clarity.

Moved the ntlm message creation and decoding from smtp.c into the sasl
module to allow for use by other modules such as pop3.

This is done introducing tags <file1> to <file4> besides existing <file> one,
as well as corresponding <stripfile1> to <stripfile4> ones, that can be used
in the <verify> section in the same way as the non-numbered ones.

Moved the login message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.

Rather than encoding the password message itself the
smtp_state_authpasswd_resp() function now delegates the work to the same
function that smtp_state_authlogin_resp() and smtp_authenticate() use
when constructing the encoded user name.

In preparation for moving to the SASL module re-factored the
smtp_auth_login_user() function to smtp_auth_login() so that it can be
used for both user names and passwords as sending both of these under
the login authentication mechanism is the same.

Moved the plain text message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.

Introduce SUPPORTCAPA and SUPPORTAUTH config commands to allow further
pop3 test server expansion for tests that require CAPA or AUTH support,
although this will need some extra work to make it fully functional.