bsdinstall/zfsboot GPT+BIOS+GELI installs now make use of GELIBOOT

In this configuration, a separate bootpool is not required.
This allows ZFS Boot Environments to be used with GELI encrypted ZFS pools.

Support for GPT+EFI+GELI is planned for the future.

Tested by: Joseph Mingrone, HardenedBSD
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D5869