Honor transited-policy-checked flag in servers

Honor transited-policy-checked flag in servers

For consistency with Heimdal and simplicity of server configuration,
do not check the transited field in krb5_rd_req() if the
transited-policy-checked flag is set in the ticket.

Add a cross-realm test using the gcred and rdreq harnesses to test
server transited processing.  Also fix the KDC capaths case so that
the client actually doesn't know the path to the server realm.  In
k5test.py, adjust _cfg_merge() to remove keys mapped to None in the
second dictionary (instead of mapping them to None in the result), so
that deleting whole sections works.  Remove the corresponding check
for None in _write_cfg_section() as it is no longer needed.

ticket: 8870 (new)
tags: pullup
target_version: 1.18