Updated the style of imap_select() before adding the LIST command.

In preparation for the addition of the LIST command, moved the mailbox
check from imap_do() to imap_select() and imap_append().

Commit 26eaa838 introduces the use of S_ISDIR() yet some compilers,
such as MSVC don't support it, so we must define a substitute using
file flags and mask.

Commit f4cc54cb (shipped as part of the 7.29.0 release) was a
bug fix that introduced a regression in that while trying to avoid
allowing directory names, it also forbade "special" files like character
devices and more. like "/dev/null" as was used by Oliver who reported
this regression.

Reported by: Oliver Gondža
Bug: http://curl.haxx.se/mail/archive-2013-02/0040.html

If the server hung up the connection without sending a closure alert,
then we'd keep probing the socket for data even though it's dead. Now
we're ready for this situation.

Bug: http://curl.haxx.se/mail/lib-2013-03/0014.html
Reported by: Aki Koskinen

Some state changes would be performed after a failure test that
performed a hard return, whilst others would be performed within a test
for success. Updated the code, for consistency, so all instances are
performed within a success test.

Some state changes would be performed after a failure test that
performed a hard return, whilst others would be performed within a test
for success. Updated the code, for consistency, so all instances are
performed within a success test.

Fixed imap_done() so that neither the FINAL states are not entered when
a custom command has been performed.

Changed imap_select_resp() to invoke imap_custom() instead of
imap_fetch() after the mailbox has been selected if a custom
command has been set.

Modified imap_perform() to start with the custom command instead of
SELECT when a custom command is to be performed and no mailbox has
been given.

Added imap_custom(), which initiates the custom command processing,
and an associated response handler imap_state_custom_resp(), which
handles any responses by sending them to the client as body data.

All untagged responses with the same name as the first word of the
custom request string are accepted, with the exception of SELECT and
EXAMINE which have responses that cannot be easily identified. An
extra check has been provided for them so that any untagged responses
are accepted for them.

Updated pop3 code following recent imap changes.

Added imap_parse_custom_request() for parsing the CURLOPT_CUSTOMREQUEST
parameter which URL decodes the value and separates the request from
any parameters - This makes it easier to filter untagged responses
by the request command.

Added custom request parameters to the per-request structure.

Minor tidy up of code layout and comments following recent changes.

Introduced the result code variable to simplify the state changes and
remove the hard returns.

For consistency changed the logic of the imap_state_append_resp()
function to test for an unsucessful continuation response rather than a
succesful one.

For consistency changed two if(constant != imapcode) tests to be
if(imapcode != constant).

Added logic in imap_perform() to perform an APPEND rather than SELECT
and FETCH if an upload has been specified.

The APPEND operation needs to be performed in several steps:
  1) We send "<tag> APPEND <mailbox> <flags> {<size>}\r\n"
  2) Server responds with continuation respose "+ ...\r\n"
  3) We start the transfer and send <size> bytes of data
  4) Only now we end the request command line by sending "\r\n"
  5) Server responds with "<tag> OK ...\r\n"

This commit performs steps 4 and 5, in the DONE phase, as more
processing is required after the transfer.

Added imap_append() function to initiate upload and imap_append_resp()
to handle the continuation response and start the transfer.

Add number of bytes retrieved from the PP cache to req.bytecount and set
req.maxdownload only when starting a proper download.

Added safer parsing of the untagged FETCH response line and the size of
continuation data.

Accidentally lost the result code in imap_state_capability() and
imap_state_login() with commit b06a7862.

Some state changes would be performed after a failure test that
performed a hard return, whilst others would be performed within a test
for success. Updated the code, for consistency, so all instances are
performed within a success test.

Small tidy up to keep some comments consistant across each of the email
protocols.

Removed superfluous NULL assignment after Curl_safefree() and rewrote
some comments and logging messages.

Following commit ad3177da corrected the debug message in state()
from AUTH to AUTH_FINAL.