Refactor mdns.MDNSService API in zone.go.

- Change API for constructing an MDNSService: use NewMDNSService func instead of
  creating an MDNSService struct and calling its Init method.
- Allow full specification of the host name and IP addresses of an MDNSService
  instead of relying on the net package to infer them from the OS.
- Remove deprecated Addr member of MDNSService.
- Modify tests to comply with new API.