init: reap zombies only after kill(-pid, ...)

When init gets SIGCHLD, it uses waitpid() to get the pid of an exited
process.  It then calls kill(-pid, ...) to ensure that all processes
in the process group started by that process are killed as well.

There is a bug here however as waitpid() reaps the pid when it
returns, meaning that the call to kill(-pid, ...) may fail with ESRCH
as there are no remaining references to that pid.  Or worse, if the
pid is reused, the wrong processes may get the signal.

This fixes the bug by using waitid() with WNOWAIT to get the pid of an
exited process, which does not reap the pid.  It then uses waitpid()
with the returned pid to do the reap only after the above kill(-pid,
...) and other operations have completed.

Bug: 38164998
Test: kill surfaceflinger and see that processes exit and are reaped
      appropriately
Test: `adb reboot` and observe that the extraneous kill() failed
      messages do not appear

Change-Id: Ic0213e1c97e0141e6c13129dc2abbfed86de138b