Replaced the hard coded md5 digest length (16) with a preprocessor
constant

It is now possible to calculate the md5 sum as the stream of buffers
becomes known where as previously it was only possible to calculate the
md5 sum of a pre-prepared buffer.

This reverts commit 9f0e1689.

It turned out that "improvement" instead made the fetching of the
certificates unreliable

Bug: http://curl.haxx.se/mail/lib-2012-03/0238.html
Reported by: Tim Heckman

This feature allows the user to specify and use additional POP3
commands such as UIDL and DELE via libcurl's CURLOPT_CUSTOMREQUEST or
curl's -X command line option.

bug: http://curl.haxx.se/mail/lib-2012-03/0278.html

bug: http://curl.haxx.se/mail/lib-2012-03/0255.html

Take in account that h_errno might be a modifiable lvalue not defined as
a C preprocessor macro

curl segfault in debug callback triggered with CURLINFO_HEADER_OUT and size 0

bug: http://curl.haxx.se/bug/view.cgi?id=3511794

Simplified the code to remove the need for a separate "LIST <msg id>"
command handler and state machine and instead use the LIST command
handler for both operations.

Corrected character and line spacing and re-ordered list and retr
functions based on the order of their state machines.

Moved the server greeting response handling code from the statemach_act
functions to separate response functions. This makes the code simpler
to follow and provides consistency with the other responses that are
handled here.

Body data would be forwarded to the client application in both the RETR
and LIST commands even if CURLOPT_NOBODY was specified.

Original wording could lead users in thinking it tries to
somehow parse the filename for a date expression (like
news_2012_03_05.html). It never mentions that it actually
reads the mtime of the file in filesystem.

This allows building of libcurl on DOS using DJGPP 2.04 and Watt-32
sockets. I know there's already Makefile.djgpp, but I find this more
convenient since I'm used to using the ./configure script from other
platforms

The commit e650dbde that stripped off [brackets] from ipv6-only host
headers for the sake of cookie parsing wrongly incremented the host
pointer which would cause a bad free() call later on.

The refactoring of HTTP CONNECT handling in commit 41b02378 that
made it protocol independent broke it for the multi interface. This fix
now introduce a better state handling and moved some logic to the
http_proxy.c source file.

Reported by: Yang Tse
Bug: http://curl.haxx.se/mail/lib-2012-03/0162.html

Take in account that POSIX standard Issue 7 drops h_errno support. Now, we also
consider getaddrinfo() to be thread-safe when (_POSIX_C_SOURCE >= 200809L) or
(_XOPEN_SOURCE >= 700) independently of whether h_errno exists or not.