Client: add two interesting function to create a ResolvSeed

* contactDNSResolver: contact a distant DNS Resolver using its
  DomainName;
* contactDNSResolverAt: contact a distant DNS Resolver using its IP
  address;

also update the makeResolvSeedSafe in order to use contactDNSResolver
Client: major improvement in makeResolvSeedSafe

* allow the use of an IP address (IPv4 or IPv6)
* now check all the available IPv4 for a given domain name
* now check all the available IPv6 for a given domain name