inbound_auth_cb(): report a "Password mismatch" error (over BinkP) if

one of the configured linked-nodes matching the remote address(es) is set for
CRAM-MD5 passwords only and a plain-text password was provided by the remote.
Previously, the function would log a warning:
"CRAM-MD5 required (and not provided) by <addr>" but succeeed (the actual
password value was not check) and it woudl flag the session as "non-secure"
(sending M_OK non-secure), looking like this on the remote side:
  Authentication successful: non-secure
and any received files would go into the configured non-secure inbound.
This may explain why some sysops sometimes get unexplained files in their
non-secure inbound (looking for "non-secure" in the logs should confirm).
Now, you should still see (on the answering side) the log message:
"CRAM-MD5 required (and not provided) ...", but it'll also send a M_ERR
Passowrd mismatch error to the remote, thus ending the session.