For reasons now lost in historical fog, the bounds_check_with_label()

For reasons now lost in historical fog, the bounds_check_with_label()
function were put in i386/i386/machdep.c from where it has been
cut and pasted to other architectures with only minor corruption.

Disklabel is really a MI format in many ways, at least it certainly
is when you operate on struct disklabel.

Put bounds_check_with_label() back in subr_disklabel.c where it belongs.

Sponsored by:   DARPA & NAI Labs.