snomasks: make +s a parametric snomask mode with the standard category letters (acdfgjklnoqrtuvwx), route each server notice by category, RPL_SNOMASKIS 008; opers default to all and narrow with +s -c etc.

src/xline.rs

@@ -261,7 +261,7 @@ impl Server {
             setter: setter.to_string(),
             expires: if duration == 0 { 0 } else { n + duration },
         });
-        self.snotice(&format!(
+        self.snotice_c('x', &format!(
             "{setter} added a {}-line on {mask}: {reason}",
             kind.tag()
         ));