Add OpenBSD 5.3, released today, May 1 2013.
In case ZFS doesn't use UMA for buffers there's no need to waste memory
creating zones that will remain empty.

Reviewed by:	pjd
Add DragonFly BSD 3.4.1, released April 29 2013.

Submitted by:	Nikolai Lifanov <lifanov@mail.lifanov.com>