split mutt_parse_mailboxes into mutt_parse_unmailboxes

split mutt_parse_mailboxes into mutt_parse_unmailboxes

Split mutt_parse_mailboxes into two functions: one to add, one to remove.
There's not much overlap in the two functions and separating them
simplifies the logic.

Where unmailboxes used to use two loops, there's now only one.