nickserv: log out the loser of a registration conflict

When a peer's registration wins a name a local session was logged into,
that session authenticated against a credential that no longer exists.
The node now clears its accountname (the ircd emits RPL_LOGGEDOUT) and
tells the user why. A new outbound channel lets the engine push these
services-initiated actions to the uplink from the gossip path, drained
by the link loop alongside incoming traffic.

modules/nickserv/nickserv.rs

@@ -21,6 +21,9 @@ impl Service for NickServ {
     fn gecos(&self) -> &str {
         "Nickname Services"
     }
+    fn manages_accounts(&self) -> bool {
+        true
+    }
 
     fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, _net: &Network, db: &mut Db) {
         let me = self.uid.as_str();