NixOS/nixpkgs — pull request #1576 by ttuegel (2014-01-23 19:09:36+00:00)
Title: Don't set shells as default whenever enabled

Because alternative shells like Zsh are almost non-functional unless
"enabled", it doesn't make sense to try setting them as default just
because they are enabled.

In other words, for Bash and Zsh, this decouples "enabled" (i.e., "usable")
from "default."