fs-util: introduce open_parent() helper

fs-util: introduce open_parent() helper

We often open the parent directory of a path. Let's add a common helper
for that, that shortens our code a bit and adds some extra safety
checks, for example it will fail if used on the root directory (which
doesn't really have a parent).

The helper is actually generalized from a function in btrfs-util.[ch]
which already existed for this purpose.