Handle authentication on subrequests

Handle authentication on subrequests

In some cases (like during directory listing) Apache will re-run the
authentication code. Many GSSAPI mechanism have replay detection so
we cannot simply rerun the accept_sec_context phase. Others require
multiple steps. When authntication has already been estalished just
implicitly consider the authentication successfully performed and
copy the user name. Otherwise fail.
If a subrequest hits a location with a different mod_auth_gssapi
configuration warn but do not error off right away.

Fixes #15