add nsupdate.info, a free dynamic dns service/software (en)

note: it should be platform-independent python code, but I only added it for the posix-compatible platforms (not for windows, as it, AFAIK, was not tested yet on windows). I also added it to the "Router" category (although the software is not intended to RUN on a router, the service is useful when setting up routers that support updating dyndns2-compatible services).