bug: broken versions of sfdisk (2.17.2)

See: https://github.com/RobertCNelson/netinstall/issues/20

Reported-by: Stephen Koone <skoone@fastmail.us>
Reported-by: Matthew Sears <msears@powermonitors.com>
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>