lib/checkout: Do UNION_FILES via atomic renameat()

lib/checkout: Do UNION_FILES via atomic renameat()

I was looking at fixing an `rpm-ostree livefs` bug where we need to replace
`/usr/lib/passwd`. It's obviously bad if that temporarily disappears 😉. My plan
is to do a subpath checkout of just `/usr/lib/{passwd,group}`.

Make this atomic (i.e. file always exists) by changing the logic to create a
temporary link in repo/tmp, then rename() it into place.

A bonus here is we kill one of the very few (only?) non-error-cleanup i.e.
"non-linear" `goto`s in the ostree codebase.

Closes: #1171
Approved by: jlebon