Work around unlinkat bug on FreeBSD and GNU/Hurd

Work around unlinkat bug on FreeBSD and GNU/Hurd

* src/unlink.c (dunlink_insert): New function.
(flush_deferred_unlinks): Skip cwds and nonempty directories
at the first pass.  If force is requested, run a second pass
removing them.
(queue_deferred_unlink): Make sure current working directory
entries are sorted in descending order by the value of dir_idx.
This makes sure they will be removed in right order, which works
around unlinkat bug on FreeBSD and GNU/Hurd.
* tests/remfiles08b.at: Remove expected failure.
* tests/remfiles09b.at: Likewise.