fixed a problem with group policy writes causing policy corruption

This bug was caused by two things:

  1) in the unix ACL mapping, we were not taking into account group
  write permssions for the SEC_STD_DELETE flag

  2) when a file is created using OVERWRITE mode, a fchmod() would
  fail if the user is not the file owner. We resolve that by only
  doing the fchmod() if the mapped file attribute does not match the
  desired file attribute