sysmopcu: Re-start the PCU on disconnect

During the refactorings I highlighted that the PCU has two
equally broken clean-up paths.

Fixes: SYS#134
diff --git a/contrib/sysmopcu.service b/contrib/sysmopcu.service
index 11e3c8c..587ca77 100644
--- a/contrib/sysmopcu.service
+++ b/contrib/sysmopcu.service
@@ -3,7 +3,7 @@
 
 [Service]
 Type=simple
-ExecStart=/usr/bin/osmo-pcu -c /etc/osmocom/osmo-pcu.cfg
+ExecStart=/usr/bin/osmo-pcu -c /etc/osmocom/osmo-pcu.cfg -e
 Restart=always
 RestartSec=5