[PATCH] use pwait in wait_for_pids

[Jilles Tjoelker]

Here is a patch to use the new pwait utility in wait_for_pids.

This patch still works if pwait is not available, using the old sleep
method.

The redirection on the pwait command serves to squelch "No such process"
and "pwait: not found" errors. The braces are necessary to redirect the
"not found" error even for sh(1) that doesn't have the fix in r197820
(which has not been MFC'ed).

-- 
Jilles Tjoelker