Authn passthru

 Make authn_passthru work again

Apache authentication methods have been broken since the website was
re-written with Django. While authn_pam handles most cases, it cannot
handle single-sign-on using an existing Kerberos TGT, but this case
is well handled by mod_auth_kerb in Apache. So, making mod_authn
function again is useful.

I've been running this under ipa for a few weeks, works fine.