Add DAVBasePath directive to allow users to configure the real repos

Add DAVBasePath directive to allow users to configure the real repos
root path, useful where the DAV repos is configured with a regex match.

* modules/dav/main/mod_dav.c
  (dav_get_resource): If available, pass the configured base path
  as the repos root to repos provider.
  On the error path for fetching a resource, detect and warn
  specifically when the location is configured via a regex.
  (dav_cmd_davbasepath): New function.

PR: 35077
Github: closes #376

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1911651 13f79535-47bb-0310-9956-ffa450edef68