Simplify shutdown process

Simplify shutdown process

On shutdown, there's no need to use kill(2) to kill the child
processes. Just closing the IPC sockets will make the children receive
an EOF, break out from the event loop and then exit.

This "pipe teardown" removes a PID reuse race condition, makes the code
simpler and allow us to remove "proc" from pledge.

OK and tweaks from claudio@