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.
Here too: "unlinked-by-default" is the (current) replacement for "keg-only".  In light of this, the line in question should be changed to something like "making this GNU Readline installation unlinked-by-default".