imap: Re-factored all perform based functions

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

imap: Added description comments to all perform based functions

imap: Removed the need for separate custom request functions

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

imap: Corrected typo in comment

imap: Moved imap_logout() to be grouped with the other perform functions

email: Updated the function descriptions for the logout / quit functions

Updated the function description comments following commit 4838d196fdbf.

email: Simplified the logout / quit functions

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

email: Simplified the logout / quit functions

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.

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email: Tidied up the *_regular_transfer() functions

Added comments and simplified convoluted dophase_done comparison.

email: Simplified nesting of if statements in *_doing() functions

imap: Fixed handling of untagged responses for the STORE custom command

Added an exception, for the STORE command, to the untagged response
processor in imap_endofresp() as servers will

imap: Fixed handling of untagged responses for the STORE custom command

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.

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imap: Fixed SELECT not being performed for custom requests

email: Minor code tidy up following recent changes

Removed unwanted braces and added variable initialisation.

email: Optimised block_statemach() functions

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

imap: Tidied up the APPEND and final APPEND response functions

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

imap: Tidied up the final FETCH response function

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

imap: Tidied up the LIST response function

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

imap: Removed the custom request response function

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

imap: Added check for empty UID in FETCH command

As the UID has to be specified by the user for the FETCH command to work
correctly, added a check to imap_fetch(), although strictly spea

imap: Added check for empty UID in FETCH command

As the UID has to be specified by the user for the FETCH command to work
correctly, added a check to imap_fetch(), although strictly speaking it
is protected by the call from imap_perform().

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imap: fix compiler warning

imap.c:694:21: error: unused variable 'imapc' [-Werror=unused-variable]

imap: Added support for list command

imap: Added list perform and response handler functions

imap: Introduced IMAP_LIST state

imap: Small tidy up of imap_select() to match imap_append()

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

imap: Moved mailbox check from the imap_do() function

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

imap: Added comments to imap_append()