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.
Hmmm...  If "Cellar path" is sufficiently intelligible, then "Cellar-path-only" is (a) sufficiently intelligible, and (b) less clunky than "unlinked-by-default".