Module devs: Add priorities for command overrides.

Use: CmdoverrideAddEx(module, name, priority, function)
Suggested by Gottem in https://bugs.unrealircd.org/view.php?id=5143
..and needed by some upcoming stuff.

include/struct.h

@@ -1704,6 +1704,7 @@ struct Command {
 
 struct _cmdoverride {
 	Cmdoverride		*prev, *next;
+	int			priority;
 	Module			*owner;
 	aCommand		*command;
 	int			(*func)();