net: stop scanning for domain name once the first label has been found

Change-Id: I95c6c85eecb53ffe52b64d521180148b624e9424
Reviewed-on: https://go-review.googlesource.com/18423Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient.go

@@ -174,6 +174,7 @@ func absDomainName(b []byte) string {
 	for _, x := range b {
 		if x == '.' {
 			hasDots = true
+			break
 		}
 	}
 	if hasDots && b[len(b)-1] != '.' {