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.
> 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 here. If you're familiar with the concept of symlinks I think this at least gives you a clue as to what we're doing. The other benefit is that use the word `linked` points towards `brew link` and `brew unlink`. I think the clunkiness is a fairly natural result of changing from a metaphor to a description; I think the more descriptive it ends up being the more clunky it will feel.
 
> 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 Then we will have two outdated sets of terminology.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.

My concern is that we won't. I think there seems to be enough opinion here that this is more newcomer friendly than keg-only (but I won't merge until you're OK with it).