ddns-scripts: add netcup.com support

ddns-scripts: add netcup.com support

Add a new netcup DDNS provider using the netcup DNS api
(ccp.netcup.net) with API key authentication.

Configuration mapping:
* username  = netcup customer number
* password  = netcup API password
* param_enc = netcup API key (generated in the CCP)
* domain    = fully qualified subdomain to update  (e.g. home.example.de)
* param_opt = (optional) root/zone domain override (e.g. example.de)
              When omitted the root domain is derived by stripping the
              leftmost label from 'domain'. This only works correctly for
              a single subdomain level (e.g. "home.example.de").
              param_opt MUST be set explicitly in two cases:
              1. Deep subdomains: domain=test.internal.example.org
              2. ccSLD apex domains: domain=example.co.nz

Signed-off-by: Tim Flubshi <redacted>