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.

The POP3 protocol doesn't really have the concept of error codes and
uses +, +OK and -ERR in response to commands to indicate continue,
success and error.

The AUTH command is one of those commands that requires multiple pieces
of data to be sent to the server where the server will respond with + as
part of the handshaking. This meant changing the values before
continuing with the next stage of adding authentication support.

Changed the order of the state machine to match the order of actual
events.

Reworked some comments and function parameter positioning that I missed
the other day.

Bug: http://lists.baseurl.org/pipermail/yum-devel/2012-January/009002.html

Since it isn't a feature supported by curl-config we can't compare that
with the --version output

Additionally corrected another RFC link that I missed yesterday.

Added support for detecting the supported SASL authentication mechanisms
via the AUTH command. There are two ways of detecting them, either by
using the AUTH command, that will return -ERR if not supported or by
using the CAPA command which will return SASL and the list of mechanisms
if supported, not include SASL if SASL authentication is not supported
or -ERR if the CAPA command is not supported. As such it seems simpler
to use the AUTH command and fallback to normal clear text authentication
if the the command is not supported.

Additionally updated the test cases to return -ERR when the AUTH command
is encountered. Additional test cases will be added when support for the
individual authentication mechanisms is added.

Moved EOB definition into header file.

Switched the logic around in pop3_endofresp() to allow for the
introduction of auth-mechanism detection.

Repositioned second and third function variables where they will fit
within the 78 character line limit.

Tidied up some comments.

Metalink file contains several hash types of checksums, such as
md5, sha-1, sha-256, etc. To deal with these checksums, I created
abstraction layer based on lib/curl_md5.h and
lib/md5.c. Basically, they are almost the same but I changed the
code so that it is not hash type dependent. Currently,
GNUTLS(nettle or gcrypt) and OpenSSL functions are supported.

Checksum checking is done by reopening download file.  If there
is an I/O error, the current implementation just prints error
message and does not try next resource.

In this patch, the supported hash types are: md5, sha-1 and sha-256.

Filenames contained in Metalink file can include directory information.
Filenames are unique in Metalink file, taking into account the directory
information. So we need to create the directory hierarchy.

Curl has --create-dirs option, but we create directory hierarchy for
Metalink downloads regardless of the option value.

This patch also put metalink int variable outside of HAVE_LIBMETALINK
guard. This reduces the number of #ifdefs.