Add isoboot(8) for booting BIOS systems from HDDs containing ISO images.

Add isoboot(8) for booting BIOS systems from HDDs containing ISO images.

This is part of a project for adding the ability to create hybrid CD/USB boot
images. In the BIOS case when booting from something that isn't a CD we need
some extra boot code to actually find our next stage (loader) within an
ISO9660 filesystem. This code will reside in a GPT partition (similar to
gptboot(8) from which it is derived) and looks for /boot/loader in an
ISO9660 filesystem on the image.

Reviewed by:	imp
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D14914