* Change KEY/HMAC_KEY to __CHANGEME__, which is rejected by fwknopd
   during start-up. The value CHANGEME is used only by LuCI package
   luci-app-fwknopd - pull request for generating keys directly from
   LuCI has been created already.
 * Add sensible defaults for ENABLE_IPT_FORWARDING and ENABLE_NAT_DNS,
   which both are/were set by luci-app-fwknopd. Move the defaults here.

Signed-off-by: Oldřich Jedlička <oldium.pro@gmail.com>