Enable online modification of servers

The address, port, monuser and monpw parameters of an existing server can
be changed at runtime. The support for enabling SSL will come in a later
commit.

Allowing servers to be modified could also be done by destroying and
recreating them. Since the servers are never actually destroyed, it is
better to allow the alteration of the existing ones.

server/modules/monitor/mysqlmon/mysql_mon.c

@@ -692,12 +692,7 @@ monitorDatabase(MONITOR *mon, MONITOR_SERVERS *database)
     unsigned long int server_version = 0;
     char *server_string;
 
-    if (database->server->monuser != NULL)
-    {
-        uname = database->server->monuser;
-    }
-
-    if (uname == NULL)
+    if (!database->server->monuser[0])
     {
         return;
     }