[capsicum] unlinkfd

[Christoph Hellwig]

With my Linux hat I'd much prefer using the AT_EMPTY_PATH flag that
Linux already supports for a few *at calls to operate on the dirfd
fd.  But it seems like neither FreeBSD nor anyone else picked up that
flag, so it might be a bit of a hard sell.

FreeBSD bugzilla related to AT_EMPTY_PATH and the lack of it for
unlinkat even in Linux:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197778

Linux bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=93441

If you are interested in bringing this to FreeBSD that might be reason
enough for me into looking into a Linux implementation as well.