This mostly involves handling GPT GUID types.  Oh, we *do* use sfdisk to
set the part type for GPT disks.  We stopped doing it for MBR disks
because there was an observed behavior that sfdisk would whack the BSD
disklabel when setting the partition type.  But, I assume we'll never
have a BSD-partitioned disk in a GPT table, given our current partition
style.

(I also added a few optional (off by default) partprobes to deal with
some funny behavior when testing (i.e., setting part types from 0 to X
to 0, over and over).  The kernel is currently kind of funny.  It
creates /dev entries for block devices that have part type 0 at boot; it
creates /dev entries for block devs that you haven't edited when you
simply runs partprobe (or leaves them intact); but if you make a type
change from 0 to X and back to 0, partprobe /dev/foo does *not* create
the device.  I'm sure this behavior has to do with the limits the kernel
will accept for making changes to a disk with mounted partitions; but it
is nonetheless strange.  Anyway, this is optional because on some
kernels, at least, a forced partprobe will result in any 0-typed
partitions not showing up in /dev, which is not very helpful.  So it's
there if it's helpful during testing, I guess.)