resolv: Make DNS-SD standards & avahi compliant

This commit updates resolv to more robustly support DNS Service
Discovery. It also is interoperable with avahi on Linux.

Changes:

- Keep explicit tables of the services the node provides, the services
  the node is trying to discover, and services that the node knows
  others provide. This is necessary because more than one device can
  support the same service.
- Update the packet formats to be standards compliant.
- Provide the "_services._dns-sd._udp.local" PTR so that avahi knows
  that the services the node has exist (and will show up in
  `avahi-browse --all`.
- Handle packets when other devices ask if the node supports a given
  service.
- Properly parse PTR, SRV, and TXT packets to discover devices that
  support a given service.

Author: bradjc