bugfix: Properly exit a process.

bugfix: Properly exit a process.

This requires that onExit() is not called immediately upon receiving a
SIGCHLD. There could still be data in the pipez. So, instead just set a
flag and invoke the pipe watchers.

Sometimes one will not receive an EOF from pipes because the process was
killed by a SIGTERM, or something. If SIGCHLD has been recved but we are
getting EAGAIN, the pipez need to be closed too.