changes to OPER command

* Impose a throttle on OPER attempts regardless of whether they caused a
  password check.
* Never disconnect the client on a failed attempt, even if there was a
  password check.
* Change error numeric to ERR_NOOPERHOST
* Explicit information about the failure in the server log (copying Insp)

Fixes #2296.

irc/client.go

@@ -189,6 +189,8 @@ type Session struct {
 	fakelag              Fakelag
 	deferredFakelagCount int
 
+	lastOperAttempt time.Time
+
 	certfp     string
 	peerCerts  []*x509.Certificate
 	sasl       saslStatus