Share the ADMIN text and KILL reason, and test the review's gaps

ADMIN's location and email lines come from service.AdminLines, and
KillUser applies the default reason, so both transports send the same
reply. The Disconnect test now allows a quarter of one write window,
so a Disconnect that writes before returning fails it. QueryUserMode
and UserhostReply are tested with a read that fails. The handler tests
again cover USERHOST for a nick with no session, MODE +o on your own
nick, and the status of a successful KILL. handleMode's comment says
what it does.

Model: opus-5-5

internal/ircserver/export_test.go

@@ -56,6 +56,10 @@ func (s *Server) Listener() net.Listener {
 	return s.listener
 }
 
+// KillWriteWindow is how long Disconnect allows each write
+// to a killed victim.
+const KillWriteWindow = killWriteWindow
+
 // NewTestConn returns a Conn for nick on tcpConn, without a
 // server, database or service behind it.
 func NewTestConn(