systemd: order qubes-mount-dirs.service before local-fs.target

The service is really responsible for mounting /rw and /home, so should
be ordered before local-fs.target - this will allow other services
to use standard ordering targets.

This probably makes Before=qubes-gui-agent.service not needed anymore,
but do not remove it yet without extensive testing to not risk
regression.

Fixes QubesOS/qubes-issues#2194

vm-systemd/qubes-mount-dirs.service

@@ -1,6 +1,8 @@
 [Unit]
 Description=Initialize and mount /rw and /home
-Before=qubes-gui-agent.service
+After=qubes-sysinit.service
+DefaultDependencies=no
+Before=local-fs.target qubes-gui-agent.service
 
 [Service]
 Type=oneshot