NixOS has a pervasive dependency on bash. For instance, the X11
session script sources /etc/profile to get a reasonable
environment. Thus we should not provide an option to disable bash.
Also, enabling zsh no longer sets ‘users.defaultUserShell’ to zsh, to
prevent a collision with bash's definition of the same
option. (Changing the default shell is also something that should be
left to the user.)
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
