Call chainevh callback when we are invoked with neither MOD_LOAD nor

MOD_UNLOAD.  This makes it possible to add custom hooks for other module
events.

Return EOPNOTSUPP when there is no callback available.

Pointed out by:	jhb
Reviewed by:	jhb
MFC after:	1 month

sys/kern/kern_syscalls.c

@@ -181,7 +181,9 @@ syscall_module_handler(struct module *mod, int what, void *arg)
 		error = syscall_deregister(data->offset, &data->old_sysent);
 		return (error);
 	default:
-		return EOPNOTSUPP;
+		if (data->chainevh)
+			return (data->chainevh(mod, what, data->chainarg));
+		return (EOPNOTSUPP);
 	}
 
 	/* NOTREACHED */