debhelper add-on to handle systemd unit files

 dh-systemd provides a debhelper sequence addon named 'systemd' and the
 dh_systemd_enable/dh_systemd_start commands.
 .
 The dh_systemd_enable command adds the appropriate code to the postinst, prerm
 and postrm maint scripts to properly enable/disable systemd service files.
 The dh_systemd_start command deals with start/stop/restart on upgrades for
 systemd-only service files.