Properly implement neg_mechs & GM_USE_MG_CRED

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.

lib/gssapi/krb5/external.c

@@ -391,8 +391,6 @@ static gssapi_mech_interface_desc krb5_mech = {
     _gsskrb5_duplicate_cred,
     _gsskrb5_add_cred_from,
     _gsskrb5_store_cred_into,
-    NULL, /* gm_set_neg_mechs */
-    NULL, /* gm_get_neg_mechs */
     NULL, /* gm_query_mechanism_info */
     NULL, /* gm_query_meta_data */
     NULL, /* gm_exchange_meta_data */