imap: Fixed memory leak when performing multiple selects

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.
pop3: Updated do phrase clean-up comment

Following commit 65644b833532 for the IMAP module updated the clean-up
comment in POP3.