Add ap_register_provider and ap_lookup_provider functions which resolve

Add ap_register_provider and ap_lookup_provider functions which resolve
the DSO link problems for DAV and the new aaa modules by moving the
provider code into the core of the server and generalizing them to be
used by any code.

Remove the auth{nz}_*_provider functions as they are no longer needed.

Change the dav_*_provider functions to wrap the ap_*_provider functions
as they have a bit more of a historical precedent that we should keep
around.

Reviewed by:	John K. Sterling <john@sterls.com> (in concept)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96919 13f79535-47bb-0310-9956-ffa450edef68