Do some preprocessing on the DNS configuration sent to plugins:

 - add the '~' default routing (lookup) domain to IP configurations
   with the default route or, when there is none, to all non-VPN
   IP configurations

 - use the dns-priority to decide which connection to use in case
   multiple connections have the same domain

 - consider a negative dns-priority value as a way to 'shadow' all
   subdomains from other connections

 - compute reverse DNS domains

and add the resulting domain list to NMDnsIPConfigData so that
split-DNS plugins can use that directly instead of reimplementing the
same logic themselves.