ensure FileFd doesn't try to open /dev/null as atomic and co

ensure FileFd doesn't try to open /dev/null as atomic and co

The wrapping will fail in the best case and actually end up deleting
/dev/null in the worst case. Given that there is no point in trying to
write atomically to /dev/null as you can't read from it again just
ignore these flags if higher level code ends up trying to use them on
/dev/null.

Git-Dch: Ignore