Set channel mode +r when a channel is registered

ChanServ sends FMODE to mark a registered channel +r (the m_services
c_registered mode), and -r on drop. Sourced from our server, which the
ircd requires for that mode, with a low TS so it applies to the existing
channel.

src/engine/service.rs

@@ -74,4 +74,12 @@ impl ServiceCtx {
             nick: nick.to_string(),
         });
     }
+
+    // Set channel modes from services, e.g. "+r" on a registered channel.
+    pub fn channel_mode(&mut self, channel: &str, modes: &str) {
+        self.actions.push(NetAction::ChannelMode {
+            channel: channel.to_string(),
+            modes: modes.to_string(),
+        });
+    }
 }