the initial implementation of file truncation.  Now both open(2)ing
      a file with the O_TRUNC flag and the {,f}truncate(2) system calls
      will resize a file appropriately.  The limitations are that only
      uncompressed and unencrypted files are supported.  Also, there is
      only very limited support for highly fragmented files (the ones whose
      $DATA attribute is split into multiple attribute extents).
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>