cmd/split: print a progress counter.

cmd/split: print a progress counter.

We don't know how many bytes we're going to split in total, but we can at
least print the total number of bytes we've seen so far.

Also fix cmd/random to *not* print progress messages by default, since my
test situation is
	bup random 100M | bup split -b
and they scribble over each other when they both print progress output.  bup
random now gets a '-v' option.

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