pop3: Moved pop3_command() to be with the other perform functions

Started to apply the same tidy up to the POP3 code as applied to the
IMAP code in the 7.30.0 release.
pop3: Moved pop3_quit() to be with the other perform functions
pop3: Added description comments to all perform based functions
pop3: Re-factored all perform based functions

Standardised the naming of all perform based functions to be in the form
pop3_perform_something() following the changes made to IMAP.