bsdinstall: hook up help line and prompt for ZFS disk selection

Previously we were passing the wrong variable names for the prompt and
help line, so the intended action wasn't clear to the user.

Reviewed by: jhb, markj
MFC after: 3 days
See also: https://github.com/opnsense/installer/issues/22
Pull Request: https://github.com/freebsd/freebsd-src/pull/1579

(cherry picked from commit d726bc286325dd7093b83c6ebcbaa768c687d825)