net: use slightly less predictable dns request id

not trying to be secure, just not repetitive
(sending with the same id repeatedly makes
some resolvers stop replying.)

eventually we'll replace this with linking against
the system's native resolver.

R=p
CC=golang-dev
https://golang.org/cl/207051

src/pkg/net/dnsclient.go

@@ -17,6 +17,8 @@ package net
 import (
 	"once"
 	"os"
+	"rand"
+	"time"
 )
 
 // DNSError represents a DNS lookup error.
@@ -44,7 +46,7 @@ func _Exchange(cfg *_DNS_Config, c Conn, name string) (m *_DNS_Msg, err os.Error
 		return nil, &DNSError{"name too long", name, ""}
 	}
 	out := new(_DNS_Msg)
-	out.id = 0x1234
+	out.id = uint16(rand.Int()) ^ uint16(time.Nanoseconds())
 	out.question = []_DNS_Question{
 		_DNS_Question{name, _DNS_TypeA, _DNS_ClassINET},
 	}