dcache-xrootd: Fix stat, fix return codes

Addresses protocol compliance issues in the xrootd implementation:

- Return kXR_NotFound if file is not found (affects stat, open, rm,
  rmdir, mv, list

- Implement kXR_offline, and kXR_poscpend in stat response. A file
  is offline if not on a read pool (ie it requires staging from tape,
  pool to pool, or that pools come online). A file is poscpend if
  currently being uploaded.

- Change kXR_writable, kXR_readable, kXR_xset of stat such that these
  are interpreted wrt the current subject, rather than just returning
  the world permissions.

- The stat changes also apply to statx.

- Change kXR_open to return kXR_FileLocked if file is currently being
  uploaded.

Target: trunk
Require-book: no
Require-notes: yes
Request: 2.6
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5817/
(cherry picked from commit daa9c9d57c40ff23e5148a344c76919f1dbe2b97)