repo: Add _ostree_repo_allocate_tmpdir helper

repo: Add _ostree_repo_allocate_tmpdir helper

This creates a subdirectory of the tmp dir with a selected prefix,
and takes a lockfile to ensure that nobody else is using the same directory.
However, if a directory with the same prefix already exists and is
not locked that is used instead.

The later is useful if you want to support some kind of resumed operation
on the tmpdir.

touch reused dirs

https://bugzilla.gnome.org/show_bug.cgi?id=757611