webdav: log why request is rejected with permission denied

Certain configuration can lead to a user's request being rejected
inside the webdav door.  These are currently either not logged or
logged with limited information.  Either way, it is hard to
understand why a request is failing.

Target: master
Request: 2.7
Request: 2.6
Request: 2.2
Patch: http://rb.dcache.org/r/6319/
Acked-by: Gerd Behrmann