Revert "Don’t assume openat"

Revert "Don’t assume openat"

This reverts commit 3cccf0a9107d585173e527550bbc45253624ca2e.

This is a change with far-reaching effects on MS-Windows at the least,
where file-related APIs are shadowed to support transparent support
for UTF-8 encoded file names.  Making such changes on a stable branch
for the benefit of a proprietary platform with a 13-year old OS is a
tail wagging the dog.  Please don't do that without discussing first.