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.
@DomT4 @adamv The problem I have is that those are all metaphors where `unlinked-by-default` is an actual description. It's clunky because the concept is complex; we symlink lots of things but for a few packages we don't do any symlinking by default. @adamv How strongly do you feel about this? Would you object to my merging and us revisiting in future?