Rethinking race-free process signaling

There's no purpose in using "short" to store PIDs when we have "pid_t" and using "char[6]" as string storage for a PID has always been a pretty bad idea. The print formatting that assumes six columns is ugly but not dangerous, and apparently people just don't care, as "vmstat" has had broken column widths for at least ten years (block IO per second and memory sizes are much larger than vmstat originally designed for).