Configure a KDC, add test for GSSAPI/Kerberos

Configure a KDC, add test for GSSAPI/Kerberos

Using nss_wrappers so we can control host names we can
setup a KDC and test GSSAPI, including fallback to
forms-based auth.

This also means that fetch_page() needs to handle 401
a bit better, so it can re-try a failed authentication or
fall back to forms-based auth.

Note for posterity: if gss_localname() fails this is likely
due to using the wrong krb5.conf in Apache, so pass in all
environment variables.

The KDC setup code was based heavily on the tests in the
gssproxy project.

https://fedorahosted.org/ipsilon/ticket/116

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>