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"

PR: 226118
Glanced at by: imp