cmd/split: fixup progress message, and print -b output incrementally.

cmd/split: fixup progress message, and print -b output incrementally.

As a side effect, you can no longer combine -b with -t, -c, or -n.  But that
was kind of a pointless thing to do anyway, because it silently enforced
--fanout=0, which is almost certainly not what you wanted, precisely if you
were using -t, -c, or -n.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>