mgcp: Group the state for bts/net into a struct and have two instances

Group the data that each end (network/bts) have into a struct and use
this struct throughout the sourcecode.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index 2546236..d309ba2 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -247,7 +247,7 @@
 			LOGP(DMGCP, LOGL_ERROR, "Can not get the peername...%d/%s\n",
 			      errno, strerror(errno));
 		} else {
-			mgcp_endp->bts = sock.sin_addr;
+			mgcp_endp->bts_end.addr = sock.sin_addr;
 		}
 
 		/* send the message and a fake MDCX to force sending of a dummy packet */