netns-isolation: netns architecture

- Adds network namespace instantiation and routing architecture.
- netns-isolation disabled by default. Can be enabled with
  configuration.nix FIXME.
- Uses mkMerge to toggle certain options for non netns and netns
  systems.
- Adds security wrapper for netns-exec which allows operator to exec
  with cap_sys_admin
- User can select the 169.254.N.0/24 addressblock netns's are created in.
- nix-bitcoin-services IpAddressAllow is amended with link-local
  addresses

examples/configuration.nix

@@ -117,6 +117,13 @@
   # `docs/usage.md`.
   # services.hardware-wallets.trezor = true;
 
+  ### netns-isolation (EXPERIMENTAL)
+  # Enable this module to use Network Namespace Isolation. This feature places
+  # every service in its own network namespace and only allows truly necessary
+  # connections between network namespaces, making sure services are isolated on
+  # a network-level as much as possible.
+  # nix-bitcoin.netns-isolation.enable = true;
+
   # FIXME: Define your hostname.
   networking.hostName = "nix-bitcoin";
   time.timeZone = "UTC";