Use lxc-stop to stop systemd service

Ever since 8eb62c2, systemd has not been able to cleanly stop lxc
containers (via lxc@) because it's still using SIGPWR for systemd-based
containers.

We should now use the nice logic in 330ae3d to stop the containers
instead.

Signed-off-by: JD Friedrikson <yours@decompo.site>

config/init/systemd/lxc@.service.in

@@ -8,9 +8,9 @@ Documentation=man:lxc-start man:lxc
 [Service]
 Type=simple
 KillMode=mixed
-KillSignal=SIGPWR
 TimeoutStopSec=120s
 ExecStart=@BINDIR@/lxc-start -F -n %i
+ExecStop=@BINDIR@/lxc-stop -n %i
 # Environment=BOOTUP=serial
 # Environment=CONSOLETYPE=serial
 Delegate=yes