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/mgcp_internal.h b/include/osmocom/mgcp/mgcp_internal.h
index 06883dd..f75ae8b 100644
--- a/include/osmocom/mgcp/mgcp_internal.h
+++ b/include/osmocom/mgcp/mgcp_internal.h
@@ -226,7 +226,7 @@
 	enum mgcp_connection_mode mode_orig;
 
 	/*! connection id to identify the connection */
-	char id[MGCP_CONN_ID_LENGTH];
+	char id[MGCP_CONN_ID_MAXLEN];
 
 	/*! human readable name (vty, logging) */
 	char name[256];