Fix handling of wrapped processes [part 1].

Change 3fe59510 changed our pipe creation to CLOEXEC,
which doesn't work so well for wrapper processes that call exec.

Sending down the PID pre-exec won't work so well because wrappers
are arbitrary and might fork themselves before finally starting
the child application process.

Change-Id: I065f0f2150dc1ff04d2b2e8b25f06b4c754809b9