GSSAPI error message improvements

GSSAPI error message improvements

Make the error messages around GSSAPI encryption a bit clearer.  Tweak
some messages to avoid plural problems.

Also make a code change for clarity.  Using "conf" for "confidential"
is quite confusing.  Using "conf_state" is perhaps not much better but
that's what the GSSAPI documentation uses, so there is at least some
hope of understanding it.