libmsc: fix: properly initialize the SGs server

It was observed that the SGs server is started before
the actual VTY configuration is parsed. For example:

  sgs
   local-port 9999
   local-ip 127.0.0.1
   vlr-name vlr.example.net

produces the following debug output:

  <0011> sgs_server.c:185 SGs socket bound to r=NULL<->l=0.0.0.0:29118
  DLSS7 NOTICE <001e> osmo_ss7.c:1284 0: ASP Restart for server not implemented yet!
  DSGS NOTICE <0011> sgs_server.c:185 SGs socket bound to r=NULL<->l=0.0.0.0:9999
  DSGS NOTICE <0011> sgs_server.c:185 SGs socket bound to r=NULL<->l=127.0.0.1:9999
  DMNCC DEBUG <0004> msc_main.c:604 Using internal MNCC handler.

The first startup is triggered by sgs_iface_init(), before reading
the VTY configuration, so the logging style is different. The next
two calls to sgs_server_open() are triggered during reading of the
VTY configuration by cfg_sgs_local_port() and cfg_sgs_local_ip().

Let's avoid starting the SGs server three times, and do it once,
after the VTY configuration is parsed. Also, keep the possibility
to change the binding parameters at run-time.

Change-Id: Ie0c31205ac48be7e50d0380a89833771b2708da4
diff --git a/src/libmsc/sgs_vty.c b/src/libmsc/sgs_vty.c
index 8ba5a79..dbf2607 100644
--- a/src/libmsc/sgs_vty.c
+++ b/src/libmsc/sgs_vty.c
@@ -56,9 +56,11 @@
 	osmo_strlcpy(sgs->cfg.local_addr, argv[0], sizeof(sgs->cfg.local_addr));
 	osmo_stream_srv_link_set_addr(sgs->srv_link, sgs->cfg.local_addr);
 
-	rc = sgs_server_open(sgs);
-	if (rc < 0)
-		return CMD_WARNING;
+	if (vty->type != VTY_FILE) {
+		rc = sgs_server_open(sgs);
+		if (rc < 0)
+			return CMD_WARNING;
+	}
 
 	return CMD_SUCCESS;
 }
@@ -74,9 +76,11 @@
 	sgs->cfg.local_port = atoi(argv[0]);
 	osmo_stream_srv_link_set_port(sgs->srv_link, sgs->cfg.local_port);
 
-	rc = sgs_server_open(sgs);
-	if (rc < 0)
-		return CMD_WARNING;
+	if (vty->type != VTY_FILE) {
+		rc = sgs_server_open(sgs);
+		if (rc < 0)
+			return CMD_WARNING;
+	}
 
 	return CMD_SUCCESS;
 }