privsep: Improve the race to exit

privsep: Improve the race to exit

Each process should now cleanly wait for child processes to exit.
They should only exit when no children left.

There is still no way to cleanly log the privilged process exiting
as well as the manager process as the manager needs the
privilged process to log.

Now, at least, dhcpcd should alway say it's exited.