During static configuration, netcfg checks if the IP address, netmask
and gateway address are valid but not the nameserver address list.
Not only invalid addresses are silently discarded but also addresses
separated with the wrong character (e.g. comma or semicolon) are silently
discarded, resulting in no nameserver options in generated network
configuration files.

So check nameserver input and prompt the user again if invalid.

Closes: #1089036