Stage #2 of aaa rewrite: Add provider support so that mod_authn_* modules

Stage #2 of aaa rewrite: Add provider support so that mod_authn_* modules
do not have to re-implement basic auth and to allow mod_auth_digest (and
other modules) to leverage the authn backends.

Adds AuthBasicProvider and AuthDigestProvider directives.

This also moves a lot of the basic auth handling code inside of mod_auth_basic
(but does not remove the code in server/protocol.c - that will have to wait
for a version bump so that we don't totally bust old modules).

This patch incorporates code review comments by Greg Stein.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96739 13f79535-47bb-0310-9956-ffa450edef68