dynamicdns: Fix writing configuration for no NAT
dynamicdns: Fix writing configuration for no NAT

- When writing the configuration file for no NAT case, append then last
  part of the file instead of overwriting.

- Also 'echo' statements are missing leading to attempt to execute the
  config options instead of writing them to a file.

freedomboxtwh/Plinth,harry-7/Plinth,vignanl/Plinth,freedomboxtwh/Plinth,vignanl/Plinth,vignanl/Plinth,harry-7/Plinth,kkampardi/Plinth,kkampardi/Plinth,vignanl/Plinth,harry-7/Plinth,vignanl/Plinth,kkampardi/Plinth,harry-7/Plinth,kkampardi/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,freedomboxtwh/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth