Revert "systemd: simplify unit file"

This reverts commit 91898aa8b0.

See additional comments in the following bug:

  https://bugzilla.gnome.org/show_bug.cgi?id=701229

Basically, 'mask' and 'unmask' operations are not the ones we should be using
or suggesting; and the Alias= for the DBus file is the correct way to go.

data/Makefile.am

@@ -33,10 +33,17 @@ endif
 
 # DBus Activation file
 dbusactivationdir = $(datadir)/dbus-1/system-services
-dbusactivation_in_files = org.freedesktop.ModemManager1.service.in
-org.freedesktop.ModemManager1.service: org.freedesktop.ModemManager1.service.in
-	$(edit) $< >$@
 dbusactivation_DATA = org.freedesktop.ModemManager1.service
+dbusactivation_in_files_systemd = org.freedesktop.ModemManager1.service.systemd.in
+dbusactivation_in_files_nosystemd = org.freedesktop.ModemManager1.service.nosystemd.in
+if HAVE_SYSTEMD
+org.freedesktop.ModemManager1.service: org.freedesktop.ModemManager1.service.systemd.in
+	$(edit) $< >$@
+else
+org.freedesktop.ModemManager1.service: org.freedesktop.ModemManager1.service.nosystemd.in
+	$(edit) $< >$@
+endif
+
 
 # Icon
 icondir=${datadir}/icons/hicolor/22x22/apps