Make spoofed disklabel boundstart and boundend default to the bounds

Make spoofed disklabel boundstart and boundend default to the bounds
of the usable LBA range defined by the GPT header. And then shrink
them to the bounds of the first OpenBSD partition if one is found.

While here simplify the logic, eliminate some superfluous variables
and reduce use of magic numbers.

Improvement suggested by sobrado@ ok kettenis@