Restore default core limit of 0:infinity

Continuation of 79c3c16dcbb3b45c0f108550cb89ccd4fc855e3b. Systemd 229
sets the default RLIMIT_CORE to infinity, causing systems to be
littered with core dumps when systemd.coredump.enable is disabled.

This restores the 15.09 soft limit of 0 and hard limit of infinity.

nixos/modules/system/boot/coredump.nix

@@ -50,6 +50,11 @@ with lib;
 
     (mkIf (!config.systemd.coredump.enable) {
       boot.kernel.sysctl."kernel.core_pattern" = mkDefault "core";
+
+      systemd.extraConfig =
+        ''
+          DefaultLimitCORE=0:infinity
+        '';
     })
   ];