systemd: Add service units for the BSC, NITB and MGCP

Add simple systemd service units.
diff --git a/openbsc/contrib/systemd/osmo-bsc-mgcp.service b/openbsc/contrib/systemd/osmo-bsc-mgcp.service
new file mode 100644
index 0000000..b5e8ec3
--- /dev/null
+++ b/openbsc/contrib/systemd/osmo-bsc-mgcp.service
@@ -0,0 +1,9 @@
+[Unit]
+Description=OpenBSC MGCP
+
+[Service]
+Type=simple
+Restart=always
+ExecStart=/usr/bin/osmo-bsc_mgcp -c /etc/osmocom/osmo-bsc-mgcp.cfg
+Restart=always
+RestartSec=2