nixos/getty: allow disabling

https://github.com/NixOS/nixpkgs/pull/480686 made getty unconditional, fixing 
several bugs and bad interactions. This imposes no runtime costs due to lazy 
systemd-based activation, but it does drag getty and its closure in to images. 
For many NixOS installations, this is fine - they want console login and such 
anyway - but it's a burden on lights-out containers aiming for minimal image 
sizes.

This change adds a new configuration knob to totally disable getty for these 
sorts of headless/lights-out/appliance-style set-ups. The option's description 
is deliberately made stern to hopefully dissuade anyone from toggling it if 
their system isn't in that class. As the option defaults to enabling getty, 
there should be no behaviour change for anyone not explicitly opting themselves 
in - hopefully with full knowledge of the consequences.

My local testing chopped a whole 15% off one of my images by being able to 
remove getty (the big contributor being util-linux and its friends), and that's 
just for one random image - others might be even bigger relative gains. I think 
that's more than enough to justify this configuration knob.

I had a look for uses of getty in NixOS to see if anywhere wanted to either 
have a hard dependency on getty and hence explicitly enable it (so that weird 
configurations get picked up at evaluation time rather than runtime) or disable 
it. The use sites fall into two categories:

0. Virtualised set-ups that wire up units to getty listening on ttyS0. These 
probably don't want a hard dependency: it's reasonable to run these lights-out, 
and just disabling getty will DTRT here.

1. `modules/profiles/headless.nix`, which disables some of the getty units. 
This _could_ instead disable getty as a whole, but that might break e.g. 
`machinectl shell`, and I can imagine set-ups where you want that even without 
console login, so I have left it alone. Maybe someone else feels differently 
and more strongly, and then that someone else can put together a PR and argue 
their case.

nixos/modules/services/ttys/getty.nix

@@ -76,6 +76,24 @@ in
         '';
       };
 
+      enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Include getty in the system.
+
+          getty is quiescent until called into action and does not have
+          runtime costs if it is not used. The benefit of disabling it is
+          in reducing closure size.
+
+          Disabling getty means that console login may not be possible,
+          `machinectl shell` and `login` may not work, and other ills.
+          It is only recommended for lights-out, headless containers,
+          appliances, and similar configurations not meant for any human
+          interaction ever.
+        '';
+      };
+
       loginProgram = mkOption {
         type = types.path;
         default = "${pkgs.shadow}/bin/login";
@@ -133,7 +151,7 @@ in
 
   ###### implementation
 
-  config = {
+  config = mkIf cfg.enable {
     # Note: this is set here rather than up there so that changing
     # nixos.label would not rebuild manual pages
     services.getty.greetingLine = mkDefault ''<<< Welcome to ${config.system.nixos.distroName} ${config.system.nixos.label} (\m) - \l >>>'';