From Gerri:

This is an optimization in TXNetSystem to avoid recording (and looping over)
too many helpers in TSystem::fHelper.

After this change, only the helper to the redirector is recorded, and the right
connection is searched inside.

Also added:
- an implementation of Unlink, which is useful to delete files on the cluster
  (activation of helper search in TUnixSystem::Unlink and TWinNTSystem::Unlink)
  Does not work (yet) with dirs, but this is a server side problem; Andy aware.
- a bit in EFileModeMask to flag 'offline' files, i.e. files existing but
  (temporarly) not available ... xrootd has the concept, for files to be
  staged, for example; the bit 0110000 was unused and it did fit, conceptually.


git-svn-id: http://root.cern.ch/svn/root/trunk@16473 27541ba8-7e3a-0410-8455-c3a389f83636