Unionfs: overhaul whiteout code

Unionfs: overhaul whiteout code

Move all whiteout functions and helpers into a separate file, replace all
embedded whiteout code with calls to helpers.  Cleanup and consolidate the
code.  This will make it easier to replace the whiteout code with a
Linux-native whiteout implementation (once available).

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>