MFC r329725, r329831

r329725:
Consolidate three copies of ZFS commands into a central location.

There's no reason to have multiple copies of lszfs and
reloadbe. Consolidate them into one location. Also ldi_get_size is the
same everywhere (except sparc64). Make it the same everywhere as the
common definition is more general and will work on spar64.

r329831:
Fix userboot w/ ZFS after r329725

r329725 cleaned up ZFS commands duplicated in multiple places, but userboot
was not setting HAVE_ZFS when MK_ZFS != "no". This resulted in a failure to
boot (as seen in PR 226118) in bhyve, with the following message:

/boot/userboot.so: Undefined symbol "ldi_get_size"