feat(jeeves): centralize firewall configuration

- split networking into interface and firewall modules
- define named service ports in one location
- remove scattered service-level firewall rules

systems/jeeves/networking/default.nix

@@ -0,0 +1,16 @@
+{
+  imports = [
+    ./firewall.nix
+    ./interfaces.nix
+  ];
+  networking = {
+    hostName = "jeeves";
+    hostId = "0e15ce35";
+    useNetworkd = true;
+    # Without this, unconfigured NICs get a catch-all DHCP network and
+    # join the LAN uninvited (how the dead atlantic card ended up
+    # ARP-poisoning 192.168.99.14). Every interface must be configured
+    # explicitly below.
+    useDHCP = false;
+  };
+}