units: make emergency.service conflict with rescue.service

They both use StandardInput=tty-force so they cannot be run
concurrently.

https://bugs.freedesktop.org/show_bug.cgi?id=82778
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=757072

units/emergency.service.in

@@ -10,6 +10,7 @@ Description=Emergency Shell
 Documentation=man:sulogin(8)
 DefaultDependencies=no
 Conflicts=shutdown.target
+Conflicts=rescue.service
 Before=shutdown.target
 
 [Service]