add DDNS key into nsupdate.pm for key secured updates

I've added the relevant code to key secure nsupdates when required / wanted (for example, in my implementation where i also have DHCP inserting DDNS entries for DHCP subnets)

it doesnt do anything fancy at the moment with seperate keys per nameserver, this would need to be written either into Nictool as an additional configuration on nameservers, or with some sort of keyfile insert based on the NS name, which would be easier to do tbh but less "clean"

For now, this will do :)

Also - I'm undecided whether having the full wall of comment text is useful or not, i can remove it if you want.