porthos: move files into 'machines' directory

But keep 'porthos.nix' at the root of the repository. I feel like it is
cleaner to keep device specific files at the root.

machines/porthos/networking.nix

@@ -0,0 +1,32 @@
+# Networking configuration
+{ ... }:
+
+{
+  networking = {
+    hostName = "porthos"; # Define your hostname.
+    domain = "belanyi.fr"; # Define your domain.
+
+
+    # The global useDHCP flag is deprecated, therefore explicitly set to false here.
+    # Per-interface useDHCP will be mandatory in the future, so this generated config
+    # replicates the default behaviour.
+    useDHCP = false;
+
+    interfaces = {
+      bond0.useDHCP = true;
+      bonding_masters.useDHCP = true;
+      dummy0.useDHCP = true;
+      erspan0.useDHCP = true;
+      eth0.useDHCP = true;
+      eth1.useDHCP = true;
+      gre0.useDHCP = true;
+      gretap0.useDHCP = true;
+      ifb0.useDHCP = true;
+      ifb1.useDHCP = true;
+      ip6tnl0.useDHCP = true;
+      sit0.useDHCP = true;
+      teql0.useDHCP = true;
+      tunl0.useDHCP = true;
+    };
+  };
+}