Move FSDISKTYPE uses from disklabel(8) invocations to vnconfig(8)

Move FSDISKTYPE uses from disklabel(8) invocations to vnconfig(8)
invocations, making the geometry information written to the
disklabel a bit more logically related to the disktab information
from whence it came. Also makes FSDISKTYPE usage consistent.

Flip the disklabel(8) invocations to the "echo '/ *'"
idiom to make it obvious that the desire is to create a single
'a' partition containing all free space.

No intentional functional change. MBRs, disklabels and newfs
outputs appear identical.

reads good to kn@