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.
