update the WebDAV versioning support:

- recognize and dispatch the latest set of DeltaV methods
- refine the CHECKOUT provider hook
- add avail_reports provider hook
- fix the "target" resolution mechanism


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86186 13f79535-47bb-0310-9956-ffa450edef68