Defer configuration by default and manage vpn-router.conf with ucf

Add platform = none as the shipped default so installing the package
changes nothing on the machine until a platform is chosen. Add a mode
setting (manual/interfaces/auto) controlling how much of the network
configuration is supplied versus detected from the system. Manage
/etc/vpn-router/vpn-router.conf with ucf instead of writing it once,
so dpkg-reconfigure can safely reapply debconf answers without
clobbering local edits. Extend NAT/forward rules to all local subnets,
not just the first.

debian-package/debian/install

@@ -3,7 +3,7 @@ src/usr/lib/vpn-router/vpnrouter.py                   usr/lib/vpn-router/
 src/usr/lib/vpn-router/simple-ca                      usr/lib/vpn-router/
 src/usr/lib/vpn-router/configure                      usr/lib/vpn-router/
 src/usr/lib/vpn-router/setup                          usr/lib/vpn-router/
-src/usr/lib/vpn-router/seed-config                    usr/lib/vpn-router/
+src/usr/lib/vpn-router/generate-config                usr/lib/vpn-router/
 src/usr/lib/vpn-router/vpnrouter_platforms/*.py       usr/lib/vpn-router/vpnrouter_platforms/
 src/usr/share/vpn-router/templates/*                  usr/share/vpn-router/templates/
 src/usr/share/doc/vpn-router/vpn-router.conf.example  usr/share/doc/vpn-router/