Add three kvno options from Heimdal kgetcred

Add three kvno options from Heimdal kgetcred

Add the flags --cached-only and --no-store, which pass the
corresponding options to krb5_get_credentials().  Add the option
--out-cache to write the retrieved credentials to a specified output
cache.

Add a Python test script for kvno command-line options, including
tests for the new options.

ticket: 8917 (new)