dns/ddclient: Add Multihostname support for netcup

[Sduniii]

This adds multiple hostnames to update for netcup provider.

As the ddclient contains a text field for several hostnames and I don't want to create them all individually, I have added the option to use this property as well. The queries are also reduced as only one login is performed, for example. It is backwards compatible and existing settings work. I have tested it and it continues to run reliably.

closes #3966