etc.nix: Fix style
Turn environment.etc into an attribute set

This provides a more convenient syntax and allows easier overriding.

For example,

  environment.etc = singleton
    { target = "vconsole.conf";
      source = vconsoleConf;
    };

can now be written as

  environment.etc."vconsole.conf".source = vconsoleConf;
environment.etc: Add convenience option 'text'

This allows writing

  environment.etc.hosts.text = "127.0.0.1 localhost";

instead of

  environment.etc.hosts.source = pkgs.writeText "hosts" "127.0.0.1 localhost";
Selectively allow /etc files to be disabled

For instance, if you don't want NixOS to emit /etc/hosts, you
can say:

  environment.etc.hosts.enable = false;