dch-gw: Initial commit

The *dch-gw* role, and the corresponding `dch-gw.yml` playbook, apply
all of the necessary configuration to the edge router on my home
network.

roles/dch-gw/files/ipv4-nat.nft

@@ -0,0 +1,8 @@
+#! /usr/sbin/nft -f
+
+table nat {
+	chain prerouting	{ type nat hook prerouting priority -100; }
+	chain input		{ type nat hook input priority 100; }
+	chain output		{ type nat hook output priority -100; }
+	chain postrouting	{ type nat hook postrouting priority 100; }
+}