Add allow::options::sasl (or require-sasl) to require SASL authentication

as suggested in https://bugs.unrealircd.org/view.php?id=5098
The allow block documentation has been updated, including an example at
the end of the page - https://www.unrealircd.org/docs/Allow_block

include/struct.h

@@ -1024,6 +1024,7 @@ struct _configflag_allow {
 	unsigned	useip :1;
 	unsigned	ssl :1;
 	unsigned	nopasscont :1;
+	unsigned	sasl :1;
 };
 
 struct _configitem_allow {