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

Add simple systemd service units.
diff --git a/openbsc/contrib/systemd/osmo-bsc.service b/openbsc/contrib/systemd/osmo-bsc.service
new file mode 100644
index 0000000..772d472
--- /dev/null
+++ b/openbsc/contrib/systemd/osmo-bsc.service
@@ -0,0 +1,10 @@
+[Unit]
+Description=OpenBSC BSC
+Requires=osmo-bsc-mgcp.service
+
+[Service]
+Type=simple
+Restart=always
+ExecStart=/usr/bin/osmo-bsc -c /etc/osmocom/osmo-bsc.cfg -s
+Restart=always
+RestartSec=2