Enforce GssapiAllowedMech over raw gssapi mechs
Enforce GssapiAllowedMech over raw gssapi mechs

Implemented by aqcuiring creds only for allowed_mechs and by
explicity adding spnego to the allowed_mechs set (while still
restricting spengo only to the allowed mechanism as before).

uhd-urz/mod_auth_gssapi,uhd-urz/mod_auth_gssapi,frenche/mod_auth_gssapi,davisd123/mod_auth_gssapi,devurandom/mod_auth_gssapi,davisd123/mod_auth_gssapi,frenche/mod_auth_gssapi,devurandom/mod_auth_gssapi,devurandom/mod_auth_gssapi,uhd-urz/mod_auth_gssapi,frenche/mod_auth_gssapi,davisd123/mod_auth_gssapi,frenche/mod_auth_gssapi