PR23359: impose security constraints on @kderef, @kregister
* parse.cxx: add privilege check for @kderef and @kregister
* testsuite/parseko/at_kderef.stp: New file to test privilege check
* testsuite/parseko/at_kregister.stp: New file to test privilege check
* testsuite/parseok/at_kderef.stp: New file to test privilege check
* testsuite/parseok/at_kregister.stp: New file to test privilege check