DHCP: docu, fixes

- doc: remove text that generally explains DHCP and not this model
- added auto interface selection (interface = default(""), chooseInterface())
- manually firing NF_INTERFACE_IPv4CONFIG_CHANGED is unnecessary,
  InterfaceTable already does that, and RoutingTable already fires
  IPv4_ROUTE_ADDED
- updateDisplayString(): display status over DHCP module, not the host