Don't use keg-only and pouring jargon.

This is unnecessarily confusing to end-users and the "fun" naming doesn't really communicate anything. @mistydemeo and I talked about this.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
Well, for starters, even without the change, this specific instance isn't even an accurate description of the cleanup command.

I agree that it's clunky, and I don't think new users are any more likely to understand "unlinked by default" than "keg-only" or any of the other proposed alternatives. They will still have to look something up (the Homebrew concept of linking and unlinking) before it will make sense.

I don't agree that merging it now and hoping we come up with something else later is a good strategy. Then we will have two outdated sets of terminology. I think we should wait until we come up with something that is a clear improvement.