attempt disabling ipv6 autoconfiguration at boot

This dosn't work as well as you might think if you didn't know that
the config semantics are weird. By setting 'default' we disable
autoconfig on interfaces that do not yet exist, but setting 'all' here
has no effect.

modules/base.nix

@@ -185,6 +185,18 @@ in
       };
     };
 
+    early.sysctl.net.ipv6.conf =
+      let
+        v = {
+          autoconf = 0;
+          accept_ra = 0;
+        };
+      in
+      {
+        default = v; # sets values for new interfaces
+        all = v; # probably does nothing
+      };
+
     filesystem = dir {
       dev =
         let