find-esp: Add openat() like helpers that operate on fds

find-esp: Add openat() like helpers that operate on fds

We also rework the internals of find-esp to work on directory file
descriptors instead of absolute paths and do a lot of general cleanups.

By passing the parent directory file descriptor to verify_fsroot_dir()
along with the name of the directory we're operating on, we can get rid
of the fallback that goes via path to open the parent directory if '..'
fails due to permission errors.