add systemd integration

add systemd integration

Add --with-systemd / --without-systemd options to check whether
libsystemd-daemon library is available and use it to report service
startup status to systemd for smbd/winbindd/nmbd and AD DC.

The problem it solves is correct reporting of the Samba services
at the point when they are ready to serve clients, important for
high availability software integration.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10517

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>