services: add finer-grained address family restrictions

Due to a possible NixOS bug, this commit has no effect on NixOS 20.09
where `RestrictAddressFamilies` is a no-op.
It's only relevant for NixOS unstable with cgroups v2.

bitcoind+zmq: instead of allowing all address families, only add the required
AF_NETLINK family.

lnd: lnd only runs a zmq client, not a server, therefore it requires
no additional address families.

lightning-pool, clightning-plugin-zmq: add AF_NETLINK.

modules/lightning-pool.nix

@@ -100,7 +100,8 @@ in {
         Restart = "on-failure";
         RestartSec = "10s";
         ReadWritePaths = cfg.dataDir;
-      } // (nbLib.allowedIPAddresses cfg.enforceTor);
+      } // (nbLib.allowedIPAddresses cfg.enforceTor)
+        // nbLib.allowNetlink; # required by gRPC-Go
     };
   };
 }