dhcp6: log XIDs when they differ

Encountering this message can be perfectly normal, e.g. in a high-availability
setup, where two DHCP servers answer to your requests.

internal/dhcp6/dhcp6.go

@@ -208,7 +208,7 @@ func (c *Client) sendReceive(packet dhcpv6.DHCPv6, expectedType dhcpv6.MessageTy
 			// XXX should this unpack relay messages and check the XID of the
 			// inner packet too?
 			if msg.TransactionID() != recvMsg.TransactionID() {
-				log.Printf("different XID")
+				log.Printf("different XID: got %v, want %v", recvMsg.TransactionID(), msg.TransactionID())
 				// different XID, we don't want this packet for sure
 				continue
 			}