Add AuthType of None to support disabling authentication.

Prevent crash when provider alias created to provider which is not
yet registered.
Migrate remaining functionality of mod_authn_default to mod_authn_core.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@709553 13f79535-47bb-0310-9956-ffa450edef68