Specifically, what I was trying to do involved (among other things that didn't cause problems) (un)mounting zfs datasets and listing zfs datasets. libzfs_core does not support either, and at least on FreeBSD, libzfs seems like it's supposed to be an internal-only library, since some of the headers included in libzfs.h aren't installed. It seems like the options for doing this are shelling out (ew) or maybe I can do something with zfs channel programs? Who knows. Definitely a frustrating situation.