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.

.gitignore

@@ -25,6 +25,8 @@ libtool
 *.tar.bz2
 *.pyc
 *.dirstamp
+org.freedesktop.ModemManager.service
+org.freedesktop.ModemManager.conf
 callouts/mm-modem-probe
 test/lsudev
 

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

data/ModemManager.service.in

@@ -10,3 +10,4 @@ StandardError=null
 
 [Install]
 WantedBy=multi-user.target
+Alias=dbus-org.freedesktop.ModemManager1.service

data/org.freedesktop.ModemManager1.service.nosystemd.in

@@ -2,4 +2,3 @@
 Name=org.freedesktop.ModemManager1
 Exec=@sbindir@/ModemManager
 User=root
-SystemdService=ModemManager.service

data/org.freedesktop.ModemManager1.service.systemd.in

@@ -0,0 +1,11 @@
+# This D-Bus service activation file is only for systemd support since
+# an auto-activated ModemManager would be quite surprising for those people
+# who have MM installed but turned off.  Thus the Exec path available to
+# D-Bus is /bin/false, but systemd knows the real Exec path due to the MM
+# systemd .service file.
+
+[D-BUS Service]
+Name=org.freedesktop.ModemManager1
+Exec=/bin/false
+User=root
+SystemdService=dbus-org.freedesktop.ModemManager1.service