* add mdns
* define mdns.Client
* fix some mdns stuff
* refine dns package for use with mdns
* remove Querier and Responder and replace with Client
* add record setting methods on dns.record to reuse record buffer
* protect against unbounded client answer growth
* round off sharp mdns edges; reduce allocs
* add mutlicast to stacks; add xnet tests for mdns
* fix documentation on acceptmulticast field
* mdns working
Pat Whittingslow