Implement gss_authorize_localname, as discussed on the kitten list,

Implement gss_authorize_localname, as discussed on the kitten list,
and make gss_userok a wrapper around it matching the Gnu GSS
prototype.  The SPI for gss_authorize_localname doesn't match the API
since we have no way of representing the contents of an internal name
to a mech at the moment.  From r24855, r24857, r24858, r24862, r24863,
r24864, r24866, r24867, and r24868 in
users/lhoward/moonshot-mechglue-fixes.

ticket: 6891

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24869 dc483132-0cff-0310-8789-dd5450dbe970