refactor: Move the enum message_type field directly into struct msgpath

'struct msgpath' is easier to work with for the upcoming IMAP feature
if the union discriminator is located inside the structure itself.
refactor: move sort_message_list to mairix.c from dirscan.c

This will avoid sorting the list more than once in case there are
multiple input folder types. It will also allow IMAP messages
(which will not be handled in dirscan.c) be sorted together with
the others.
Introduced MTY_IMAP message type. Not yet used.
Added code for IMAP feature
Added IMAP support in rc file and database update
Added IMAP support in searching
Added support for search results in an IMAP folder
Documented IMAP feature
RFC822 parsing: include terminating colon in header name checks

This avoids, for example, taking Subject-Monster as the Subject header.
Previously, any header name matching a prefix of the desired header
was matched.