Clear forwardable flag instead of denying request

Clear forwardable flag instead of denying request

If the client requests a forwardable or proxiable ticket and the
option cannot be honored by policy, issue a non-forwardable or
non-proxiable ticket rather than denying the request.

Add a test script for testing KDC request options and populate it with
tests for the forwardable and proxiable flags.

ticket: 7871