fs_util: recover from permission errors when removing buck-out paths

[daandemeyer]

When building OS images with buck2, read-only directories show up in
buck-out because various distributions give their top level directories
and directories in /etc restrictive permissions. Removing such outputs
fails with EACCES, since unlinking a directory entry requires write and
execute permission on the containing directory.

Mirror the existing Windows readonly fallback on Unix:

• remove_dir_all() now adds owner read/write/execute permission on
  every directory in the tree and retries when it hits a permission
  error. It only ever modifies directories inside the tree being
  removed; in particular it never touches the parent directory of the
  path passed in.

• A new remove_file_harder() adds owner read/write/execute permission
  on the containing directory and retries. Since that side effects the
  permissions of the (surviving) parent directory, it is kept separate
  from remove_file() and only used where the surrounding tree is being
  deleted anyway: the parallel file removal pass of buck2 clean and
  cleanup_path().

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D110600034. (Because this pull request was imported automatically, there will not be any future comments.)