os: make openFdAt act like openFileNolog

- add EINTR loop on Darwin
- return PathError on error
- call newFile rather than NewFile

This tries to minimize the possibility of any future changes.
It would be nice to put openFdAt in the same file as openFileNolog,
but build tags forbid.

Updates #29983

Change-Id: I866002416d6473fbfd80ff6ef09b2bc4607f2934
Reviewed-on: https://go-review.googlesource.com/c/160181


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>