internal/clientinfo: avoid heap alloc with mdns read loop

Once resource record (RR)  was used to extract necessary information, it
should be freed in memory. However, the current way that ctrld declare
the RRs causing the slices to be heap allocated, and stay in memory
longer than necessary. On system with low capacity, or firmware that GC
does not run agressively, it may causes the system memory exhausted.

To fix it, prevent RRs to be heap allocated, so they could be freed
immediately after each iterations.

internal/clientinfo/mdns.go

@@ -165,7 +165,7 @@ func (m *mdns) readLoop(conn *net.UDPConn) {
 		}
 
 		var ip, name string
-		rrs := make([]dns.RR, 0, len(msg.Answer)+len(msg.Extra))
+		var rrs []dns.RR
 		rrs = append(rrs, msg.Answer...)
 		rrs = append(rrs, msg.Extra...)
 		for _, rr := range rrs {