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.
sync_mailbox() didn't consider MaxSize == 0 to mean "unlimited".
load_config() needs to print a newline in its error messages since
next_arg() kills the newline of the line that was read out of the config
file.
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.
configuration directives
added `Maildir' configuration command to specify the default location of the
user's mailboxes. If a relative path is used in a `Mailbox' command, this
path is used as a prefix.
