Code review comment for lp:~afrantzis/unity-system-compositor/no-external-spinner-zombie-processes

> Simply calling wait() or waitpid() at the right times will avoid zombies.
> That's what "zombie process" means -- that the parent has not called wait() yet.

That was my initial approach, but the problem with this is that waitpid() blocks USC for a little bit in the best case, and can block forever if for whatever reason the external spinner happens to not respond to our signals (e.g. it has hung).

The proposed way is indeed more complicated, but it avoid all blocking problems by transferring the responsibility to the init process.

Note that a lot of the code in this MP is just test improvements, and signal-safe wrappers around read/write.