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/include/osmocom/mgcp_client/mgcp_client.h b/include/osmocom/mgcp_client/mgcp_client.h
index 6c478e8..79f2078 100644
--- a/include/osmocom/mgcp_client/mgcp_client.h
+++ b/include/osmocom/mgcp_client/mgcp_client.h
@@ -60,7 +60,7 @@
 	int response_code;
 	mgcp_trans_id_t trans_id;
 	char comment[MGCP_COMMENT_MAXLEN];
-	char conn_id[MGCP_CONN_ID_LENGTH];
+	char conn_id[MGCP_CONN_ID_MAXLEN];
 	char endpoint[MGCP_ENDPOINT_MAXLEN];
 };