We previously returned EPERM to prevent the kernel from
blacklisting our xattr support once we get an unsupported
flag, but this causes lots of trouble on MacOS:
Cannot save files from GUI apps, see
https://github.com/rfjakob/gocryptfs/issues/229
Returning ENOSYS triggers the dotfiles fallback on MacOS
and fixes the issue.
OpenDir and ListXAttr skip over corrupt entries,
readFileID treats files the are too small as empty.
This improves usability in the face of corruption,
but hides the problem in a log message instead of
putting it in the return code.
Create a channel to report these corruptions to fsck
so it can report them to the user.
Also update the manpage and the changelog with the -fsck option.
Closes https://github.com/rfjakob/gocryptfs/issues/191
