Qualify short hostnames when not using DNS

Qualify short hostnames when not using DNS

When DNS forward canonicalization is turned off or fails, qualify
single-component hostnames with the first DNS search domain.  Add the
qualify_shortname relation to override this suffix.

For one of the tests we need to disable qualification, which is
accomplished with an empty value.  Adjust k5test.py to correctly emit
empty values when writing profiles.

ticket: 8855 (new)