Add mdns package and Client implementation (#55)

* 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

http/httpraw/parse.go

@@ -295,12 +295,14 @@ func (h *Header) reuseOrAppend(tok headerSlice, value string) headerSlice {
 }
 
 func (h *Header) appendSlice(value string) headerSlice {
+	debuglog("http:appendslice:start")
 	free := h.hbuf.free()
 	if len(value) > free {
 		if h.flags.hasAny(flagNoBufferGrow) {
 			h.flags |= flagOOMReached
 			return headerSlice{}
 		}
+		debuglog("http:appendslice:grow-buf")
 		h.hbuf.buf = slices.Grow(h.hbuf.buf, len(value)+1) // Grow 1 beyond due to slice validity.
 	}
 	h.flags |= flagMangledBuffer