lib: Hoist unlinkat() cleanup API to fsutil, use in pull

lib: Hoist unlinkat() cleanup API to fsutil, use in pull

The pull code also could make use of this in both the metadata and content
paths. I changed it to own the tempfile malloc (just like `GLnxTmpFile`), since
there's no reason to have different lifetimes for the filename and the file, and
that way we only have one variable rather than two.

The content path turns out to be a special case though, where
at least for mirroring archives, we directly pass the file *path*
down into `_ostree_repo_commit_loose_final()`.

This is prep for `GLnxTmpFile` porting.

Closes: #957
Approved by: jlebon