gss: merge Apple name helper APIs

Add helper APIs to the mechanism glue from the Apple implementation.
gss_mg_export_name() can be used by a mechanism to create an exported name
(this is used by the future SAnon implementation). _gss_mech_import_name()
allows the mechanism to pass in a list of parsing functions for specific name
tiypes. This commit only adds support to the mechanism glue itself.

lib/gssapi/mech/gss_get_name_attribute.c

@@ -57,6 +57,8 @@ gss_get_name_attribute(OM_uint32 *minor_status,
     if (input_name == GSS_C_NO_NAME)
         return GSS_S_BAD_NAME;
 
+    _gss_mg_check_name(input_name);
+
     HEIM_TAILQ_FOREACH(mn, &name->gn_mn, gmn_link) {
         gssapi_mech_interface m = mn->gmn_mech;