SPNEGO was already using union creds. Now make the mechglue know about it, delete all of the cred-related SPNEGO stubs that are now not called (lib/gssapi/spnego/cred_stubs.c), and implement gss_get/set_neg_mechs() by storing the OID set in the union cred. This commit was essentially authored as much if not more by Luke Howard <lukeh at padl.com> as much as by the listed author.