mod_auth_basic: Add a generic mechanism to fake basic authentication

using the ap_expr parser. This allows the administrator to construct
their own username and password for basic authentication based on their
needs. Alternative fix for PR52616.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1457471 13f79535-47bb-0310-9956-ffa450edef68