bsdinstall zfsboot: Add an option to edit the ZFS pool creation options

This allows the default options (-O compress=lz4 -O atime=off) to be
overridden, before the ZFS boot pool is created. For example, to set the
compression algorithm to something different.

Reviewed by: jhb, dim
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D47478

(cherry picked from commit ca7e12f7df222458ec5cac8009c5b0931fb4a95b)