... hopefully this will also make clang-analyzer stop warning on
potentional NULL dereferences (which were false positives anyway).

When Curl_do() returns failure, the connection pointer could be NULL so
the code path following needs to that that into account.

Bug: http://curl.haxx.se/mail/lib-2013-03/0062.html
Reported by: Eric Hu

Standardised the naming of all perform based functions to be in the form
imap_perform_something().

No more use exit(3) but instead tell libcurl that no byte has been
written to let it return a `CURLE_WRITE_ERROR`. In addition, check
curl easy handle return code.

8 more bugfixes, one change and a bunch of contributors

Moved the custom request processing into the LIST command as the logic
is the same.

Updated the function description comments following commit 4838d196.

Moved the blocking state machine to the disconnect functions so that the
logout / quit functions are only responsible for sending the actual
command needed to logout or quit.

Additionally removed the hard return on failure.

Added comments and simplified convoluted dophase_done comparison.

Added an exception, for the STORE command, to the untagged response
processor in imap_endofresp() as servers will back respones containing
the FETCH keyword instead.

GCC specific adjustments:

- check __ILP32__ before 32 and 64bit processor architectures in
  order to detect ILP32 programming model on 64 bit processors
  which, of course, also support LP64 programming model, when using
  gcc 4.7 or newer.

- keep 32bit processor architecture checks in order to support gcc
  versions older than 4.7 which don't define __ILP32__

- check __LP64__ for gcc 3.3 and newer, while keeping 64bit processor
  architecture checks for older versions which don't define __LP64__

Bug: http://curl.haxx.se/bug/view.cgi?id=1205
Reported by: technion

Removed unwanted braces and added variable initialisation.

Optimised the result test in each of the block_statemach() functions.

Added examples of the list command and clarified existing example URLs
following recent changes.

Updated the imap features list, corrected a typo in the smtp features
and clarified a pop3 feature.

The list of unsafe functions currently consists of sprintf, vsprintf,
strcat, strncat and gets.

Subsequently, some existing code needed updating to avoid warnings on
this.

With all the recent imap changes it wasn't clear what new features and
fixes should be included in the release notes.

Just a folder list would be retrieved if UID was not specified now.

Commit 198012ee inadvertently broke LIST_imap().

Removed unnecessary state changes on failure and setting of result codes
on success.

Removed unnecessary state change on failure and setting of result code on
success.

Reworked comments as they referenced custom commands, removed
unnecessary state change on failure and setting of result code on
success.

Removed imap_state_custom_resp() as imap_state_list_resp() provides the
same functionality.