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/mech/gss_mech_switch.c

@@ -397,6 +397,8 @@ _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);
 
 		mi = (_gss_mo_init *)dlsym(so, "gss_mo_init");