Repair kill(2) on zombie processes.

Repair kill(2) on zombie processes.

kill(2) is supposed to find zombie processes, this probably got broken
when the process reaper was introduced.  As a temporary(tm) workaround,
walk the list of zombie processes if we can't find the target pid in the
main process list.

Problem with zsh initially reported by Geoff Wozniak on misc@, analysis
done by naddy@.  ok kettenis@ tedu@