Add a new installation type to bsdinstall/zfsboot: BIOS+UEFI

Installs both pmbr+gptzfsboot as well as boot1.efifat in separate partitions
The resulting system can be booted with either UEFI or BIOS/CSM.
Preference is controlled by the user's firmware boot settings.

This is now the default for zfsboot installs

PR: 208629
Submitted by: Galael LAPLANCHE <ganael.laplanche@corp.ovh.com> (original version)