support GSSAPIAuth back

GSSAPIAuthentication has not been supported
due to PreferredAuthentications trick ( 2bdba17a85 )
Also add back gssapi-keyex

While this is not currently implemented in openssh as far as I know,
there is patch floating around on the web and this may land one day
upstream.
Merge pull request #4049 from mscherer/fix_no_kerberos

Fix no kerberos