mdns: add the new openwrt mdns daemon

mdns: add the new openwrt mdns daemon

this is still wip, you can use the following ubus calls.

ubus call mdns scan	# triggers a scan
ubus call mdns browse	# look at the currenlty cached records
ubus call mdns hosts	# look at the currenlty cached hosts

TODO
- ipv6, currenlty AAAA records are handled but only on v4 sockets
- finish the service announce code

Signed-off-by: John Crispin <blogic@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41345 3c298f89-4303-0410-b956-a3cf2f4a3e73