[mdns] Add bin/dev_finder command

This adds device discovery tools to the SDK that use mDNS.

Commands include:
--resolve Looks up a device based on its hostname w/ ".local" on the
end.

--list Looks up all devices that are running the _fuchsia._udp.local
service and lists then.

TEST: Manually with a Fuchsia device using edgerouter

DX-655 #comment

Change-Id: Ibc9411d3dff51e85522a9de6ea9a481defcd5a81
6 files changed