We can get rid of the janky wannabe
self-using-a-directory-instead-of-pipe thing we needed to
workaround Filesys::Notify::Simple being blocking.
For existing Maildir users, this should be more robust and
immune to missed wakeups for signalfd and kqueue-enabled
systems; as well as being immune to BOFHs clearing $TMPDIR
and preventing notifications from firing.
The IMAP IDLE code still uses normal Perl signals, so it's still
vulnerable to missed wakeups. That will be addressed in future
commits.