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/mech/gss_mech_switch.c

@@ -417,8 +417,6 @@ _gss_load_mech(void)
 		OPTSYM(duplicate_cred);
 		OPTSYM(add_cred_from);
 		OPTSYM(store_cred_into);
-		OPTSYM(set_neg_mechs);
-		OPTSYM(get_neg_mechs);
 		OPTSPISYM(authorize_localname);
 		OPTSPISPISYM(query_mechanism_info);
 		OPTSPISPISYM(query_meta_data);