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.

Parse downloaded Metalink file and add downloads described there. Fixed
compile error without metalink support.

Don't update config->metalinkfile_last in operate(). Use local variable
to point to the current metalinkfile.

Print message if --metalink is used while metalink support is not
enabled. Migrated Metalink support in tool_operate.c and removed
operatemetalink().

This change adds experimental Metalink support to curl.
To enable Metalink support, run configure with --with-libmetalink.
To feed Metalink file to curl, use --metalink option like this:

  $ curl -O --metalink foo.metalink

We use libmetalink to parse Metalink files.

Updated various references of real domain names to example.com as per
RFC-2606.

Setting bit 2 for this value was documented as having a constant value
defined as CURL_REDIR_POST_303 yet referenced a 302 request.

Additionally corrected the meaning of CURL_REDIR_POST_ALL for all three
bits and fixed problems with the bolding of keywords in this section.

Standardised how RFCs are referenced so that the website may autolink to
the correct documentation on ietf.org. Additionally removed the one link
to RFC3986 on curl.haxx.se.

To achieve this, first new structure HeaderData is defined to hold
necessary data to perform header-related work.  Then tool_header_cb now
receives HeaderData pointer as userdata.  All header-related work
(currently, dumping header and Content-Disposition inspection) are done
in this callback function.  HeaderData.outs->config is used to determine
whether each work is done.

Unit tests were also updated because after this change, curl code always
sets CURLOPT_HEADERFUNCTION and CURLOPT_HEADERDATA.

Tested with -O -J -D, -O -J -i and -O -J -D -i and all worked fine.

Move the SMTP_AUTH constants into a separate header file in
preparation for adding SASL based authentication to POP3 as the two
protocols will need to share them.

Removed DIGEST-MD5 from Section 9.1 Other authentication mechanisms as
the feature was added to SMTP in 7.26.0.

Also corrected small spelling mistake.

Just before 7.26.0 is about to ship