Don't rely on the disklabel defaults or a disktab entry's ba#/fa#

Don't rely on the disklabel defaults or a disktab entry's ba#/fa#
values for the block/fragment sizes that some install media need.
Hoist the desired values into the newfs(8) invocations to make it
obvious non-default values are required and what the values are.

No functional change.

ok kn@