mgcp_common: rename to MGCP_CONN_ID_MAXLEN

So far, MGCP_CONN_ID_LENGTH was often used as exactly the length of the
Connection Identifier. To indicate this length as a maximum, introduce the
MGCP_CONN_ID_MAXLEN and use it everywhere. Keep the old name as an alias.

Change-Id: I1117003c7614e98535d5c201d002e459c01bdc3f
diff --git a/src/libosmo-mgcp/mgcp_conn.c b/src/libosmo-mgcp/mgcp_conn.c
index 3a5db0f..2a45b81 100644
--- a/src/libosmo-mgcp/mgcp_conn.c
+++ b/src/libosmo-mgcp/mgcp_conn.c
@@ -78,7 +78,7 @@
 		/* ensure that the generated conn_id is unique
 		 * for this endpoint */
 		if (!mgcp_conn_get_rtp(endp, id_hex)) {
-			osmo_strlcpy(id, id_hex, MGCP_CONN_ID_LENGTH);
+			osmo_strlcpy(id, id_hex, MGCP_CONN_ID_MAXLEN);
 			return 0;
 		}
 	}