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.

default.yaml

@@ -358,6 +358,10 @@ server:
     secure-nets:
         # - "10.0.0.0/8"
 
+    # allow attempts to OPER with a password at most this often. default to
+    # 10 seconds when unset.
+    oper-throttle: 10s
+
     # Ergo will write files to disk under certain circumstances, e.g.,
     # CPU profiling or data export. by default, these files will be written
     # to the working directory. set this to customize: