Add -- to cp, mv and rm

I think these are all the instances where we use cp, mv and rm on user supplied paths, but I want someone else to confirm that as well.

Can someone who regularly uses the more "exotic" shells confirm if they support this syntax and if it's even necessary? They should, but let's make sure. Particularly in regard to quitcd.

I don't foresee any regressions, but I haven't done any testing as of yet.