Floppies don't have bioctl(8)

Floppies don't have bioctl(8)

The installer always checks for softraid using it, skip if unavailable.

Apply the usual idiom in encrypt_root() to silence stderr noise.

Do so in do in get_softraid_chunks() as well which is always called in
get_dkdevs_unitialized() and finish_up();  get_softraid_chunks() discards
stderr and both users still do the right thing on bioctl failure/empty
output from get_dkdevs_unitialized(), but there's no point in trying plus
the idiom clarifies how this code is indeed used on floppies.

Found and tested by krw
OK deraadt krw