MFC r319897-r319898, r319904: Improve yes' throughput

r319897: Improve yes' throughput

On my system, this brings up the throughput from ~20 to ~600 MiB/s.

Inspired by:

https://www.reddit.com/r/unix/comments/6gxduc/how_is_gnu_yes_so_fast/

r319898: Handle partial writes

r319904: style(9) fixes.