ls -h support.

ls -h support.

It turns out that people are anticipating the switch to toybox ls from
toolbox ls on the assumption that this will finally bring -h support.
Let's not disappoint.

I've merged two existing buffers in listfiles whose uses didn't
overlap into one. It may be possible to cram this into toybuf with the
other stuff, but it didn't seem worth the extra complexity for 64B of
stack, especially since we were already living with that for time
formatting anyway.