[Toybox] buffer sizes

[enh]

just fyi if you don't follow the coreutils list, i see that they're
looking at moving up from 128KiB to 256KiB buffers (but without saying
how _much_ "more performance" that gets them, nor what exactly "modern
hardware" means).

(don't get me wrong --- this is definitely a tricky one. bionic and
musl chose smaller-than-traditional values for BUFSIZ for a reason,
and although there's a question of whether that applies to a small
stand-alone tool like toybox, i'm unconvinced that "one size fits all
for toybox" either. there's a world of difference between your minimal
mmu-less targets and even the lowest-spec'ed Android Go phone you
could ship today, and another couple of orders of magnitude between
that and a flagship Android phone. let alone the build servers
building AOSP :-) )