1. Make addDnsServer not add duplicate servers and return a
boolean value incating whether it changed anything. This is
consistent with what we do for LinkAddresses and routes.
2. Add a setDnsServers method that sets all the DNS servers to
the specified collection. This is consistent with what we do
for LinkAddress.
Bug: 9180552
Change-Id: I5baed09253261b66ea42ae2ea82398118e3ab0ac