Centralized pid handling into WaitPgrp.

To be able to create a critical section in WaitPgrp to protect
against signal races the pid handling has been moved into
WaitPgrp.

If a child process dies, its pid is set to 0 directly in WaitPgrp
which means that a pointer has to be supplied when calling the function.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>