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/templates/masquerade.nft.j2

@@ -0,0 +1,5 @@
+table ip nat {
+    chain postrouting {
+        oif {{ ansible_default_ipv4.interface }} masquerade
+    }
+}