Add zfsboot module as an option for automatic configuration. Default is

Add zfsboot module as an option for automatic configuration. Default is
to run interactively but it can be scripted too (optinally completely
non-interactive). Currently supports GELI and all ZFS vdev types. Also
performs validation on selections/settings providing error messages if
necessary, explaining (in plain language) what the issue is. Currently
the auto partitioning of naked disks only supports GPT and MBR (VTOC8
pending for sparc64), so is only available for i386/amd64 install.

Submitted by:	Allan Jude <freebsd@allanjude.com>, myself
Reviewed by:	Allan Jude <freebsd@allanjude.com>
Approved by:	re (glebius)