the uid for each message in the maildir is now stored in a dbm database
rather than the filename. this change was necessary because isync became
confused if you copied a message to another folder, in which case the uid
was invalid.
as a result of the above change, isync now acquires a mutex on the mailbox
to protect the dbm database from concurrent access.
main() was reworked to continue gracefully when an error is encountered, and
to always call maildir_close() so that the lock can be disabled, and the
database closed.
out if new mail arrives while in the process of downloading
noted in BUGS section of man page that if new mail arrives after the initial
message list has been retrieved from the IMAP server, that new mail will not
be fetched until the next invocation of isync.
