gssapi: implement gss_set_neg_mechs() (#495)

Implementation of gss_set_neg_mechs() and gss_get_neg_mechs() as defined in RFC
4178. New gss_release_cred_by_mech() API for dropping a credential from a
mechanism glue credential.

lib/gssapi/spnego/external.c

@@ -149,6 +149,8 @@ static gssapi_mech_interface_desc spnego_mech = {
     NULL, /* gm_duplicate_cred */
     gss_add_cred_from,
     NULL, /* gm_store_cred_into */
+    _gss_spnego_set_neg_mechs,
+    _gss_spnego_get_neg_mechs,
     NULL  /* gm_compat */
 };