Determine registered authn/z providers directly in ap_setup_auth_internal(),

Determine registered authn/z providers directly in ap_setup_auth_internal(),
which allows optional functions that just wrapped ap_list_provider_names()
to be removed from authn/z modules.

This change requires modules/aaa/mod_auth.h to be included into
server/request.c, which necessitates a minor change to configure.in for
Unix platforms.

I'm unable to tell whether a similar change is necessary for Windows and
NetWare builds or not.  Could developers with access to those platforms
please test and make any needed configuration or build alterations?  Thanks!

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@659160 13f79535-47bb-0310-9956-ffa450edef68