#2 unix domain socket should be deleted on clean exit

[Todd Vierling]

Reported by Bart Schaefer <schaefer@zanshin.com>:

If SIGINT/SIGTERM is received and a unix domain socket
is in use, the socket should be deleted so that it's
not on the filesystem at exit. This corresponds to
what libmilter does.

This does not invalidate the unlink check at start,
which is still quite useful in the case of a sudden die
of the milter process and/or a system reboot, where the
stray socket inode is still hanging around but is unbound.