This fixes `fg` and `bg` causing the shell to go into an infinite loop
of trying to `waitpid` until some current job changes state.
a.k.a. "Fix Shell backgrounding, yet again!" :P
Non-interactive shells (i.e. when running scripts) do not need this
functionality, so they are a boatload of wasted time.
This significantly reduces the script startup and shutdown times when
there are lots of executables in PATH or lots of entries in the history.