Introduced quite some significant changes:

* Commented out rdate NTP update line (not available on
  OpenWrt 14.07?)
* Get WAN IP via network_get_ipaddr by default,
* Divide script into Config / Action sections,
* Use UPPERCASE shell script variables,
* Split 'wan' variable up into 'DEVICE' and 'INTERFACE':
  INTERFACE should be wan on OpenWrt,
  DEVICE could be pppoe-wan if you connect via DSL (needed for IP detection fallback),
* Remove "magic" dot at the end of zone / hostname in nsupdate commands.