Corrected layout issues with the POP3 and IMAP URL examples introduced
in commit cb3ae689.

Updated the POP3 sub-section to refer to message ID rather than mailbox.

Added an IMAP sub-section with example URLs depicting the specification
of mailbox, uid and section.

Updated the mailbox variable to correctly reflect it's purpose. The
name mailbox was a leftover from when IMAP and POP3 support was
initially added to curl.

Updated the FETCH command to send the UID and SECTION parsed from the
URL. By default the BODY specifier doesn't include a section, BODY[] is
now sent whereas BODY[TEXT] was previously sent. In my opinion
retrieving just the message text is rarely useful when dealing with
emails, as the headers are required for example, so that functionality
is not retained. In can however be simulated by adding SECTION=TEXT to
the URL.

Also updated test801 and test1321 due to the BODY change.

Removed user and passwd from the SMTP struct as these cannot be set on
a per-request basis and are leftover from legacy FTP code.

Changed some comments still using FTP terminology.

Moved the rcpt variable from the per-connection struct smtp_conn to the
new per-request struct and fixed references accordingly.

Created a new SMTP structure and changed the type of the smtp proto
variable in connectdata from FTP* to SMTP*.

error: declaration of 'pop3' shadows a previous local

Updated the imap_parse_url_path() function to parse uidvalidity, uid and
section parameters based on RFC-5092.

Added imap_is_bchar() for testing if a given character is a valid bchar
or not.

Added uidvalidity, uid and section variables to the per-request IMAP
structure in preparation for upcoming URL parsing.

Removed user and passwd from the POP3 struct as these cannot be set on
a per-request basis and are leftover from legacy FTP code.

Changed some comments still using FTP terminology.

Moved the mailbox and custom request variables from the per-connection
struct pop3_conn to the new per-request struct and fixed references
accordingly.

Created a new POP3 structure and changed the type of the pop3 proto
variable in connectdata from FTP* to POP*.

Used imap_atom() to escape mailbox names in imap_select().

Moved the ftp transfer structure into pingpong.h so other protocols that
require it don't have to include ftp.h.

Corrected comment for opt_no_body variable to CURLOPT_NOBODY.

Changed some variables and comments still using FTP terminology.

Removed user and passwd from the IMAP struct as these cannot be set on
a per-request basis and are leftover from legacy FTP code.

Created a new IMAP structure and changed the type of the imap proto
variable in connectdata from FTP* to the new IMAP*.

Moved the mailbox variable from the per-connection struct imap_conn to
the new per-request struct and fixed references accordingly.

Following commit 65644b83 for the IMAP module updated the clean-up
comment in POP3.

Moved the clean-up of the mailbox variable from imap_disconnect() to
imap_done() as this variable is allocated in the do phase, yet would
have only been freed only once if multiple selects where preformed
on a single connection.

Fixes a typo get → git in docs/CONTRIBUTE.

Ignores the patch files generated by the 'git format-patch' command.

* Elaborates on default values of some curl_easy_setopt() options.
* Reminds the user to cast variadic arguments to curl_easy_setopt() to
  'void *' where curl internally interprets them as such.
* Clarifies the working of the CURLOPT_SEEKFUNCTION option for
  curl_easy_setopt().
* Fixes typo 'forth' → 'fourth'.
* Elaborates on CURL_SOCKET_TIMEOUT.
* Adds some missing periods.
* Notes that the return value of curl_version() must not be passed to
  free().

Always interprets the pointer passed with the CURLOPT_WRITEDATA or
CURLOPT_READDATA options of curl_easy_setopt() as a void pointer in
order to avoid problems in environments where FILE and void pointers
have non-trivial conversion.

* Adds several links to documentation of library functions which were
  missing.
* Marks documentation of deprecated library functions "(deprecated)".
* Removes spurious .html suffixes.

Use Curl_pp_moredata() in Curl_pp_multi_statemach() to check if there is
more data to be received, rather than the socket state, as a task could
hang waiting for more data from the socket itself.

Fixed an incorrect variable reference which was introduced in commit
a1701eea as a result of a copy and paste from SMTP/POP3.

A simple function to test whether the PP is not sending and there are
still more data in its receiver cache. This will be later utilized to:

1) Change Curl_pp_multi_statemach() and Curl_pp_easy_statemach() to
   not test socket state and just call user's statemach_act() function
   when there are more data to process, because otherwise the task would
   just hang, waiting for more data from the socket.

2) Allow PP users to read multiple responses by looping as long as there
   are more data available and current phase is not finished.
   (Currently needed for correct processing of IMAP SELECT responses.)

I just noticed Metalink support wasn't listed as a feature of the tool.