acls: Add a default-deny option, which allows you to specify what an empty ACL means.

When this option is specified, an empty ACL means "reject all clients". Without it,
an empty ACL means "accept all clients"

src/params.h

@@ -34,6 +34,8 @@ struct control_params {
 struct mode_serve_params {
 	/** address/port to bind to */
 	union mysockaddr     bind_to;
+	/** does an empty ACL mean "deny all"? */
+	int                  default_deny;
 	/** number of entries in current access control list*/
 	int                  acl_entries;
 	/** pointer to access control list entries*/