imap: Introduced a custom IMAP structure for per-request data

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.
imap: Removed some FTP heritage leftovers

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.
imap: Removed more FTP leftovers

Changed some variables and comments still using FTP terminology.