net: make lookupPort case-insensitive on Plan 9

The CL 28951 added TestLookupPort_Minimal, which was failing
on Plan 9, because lookupPort was not case-insensitive.

Change-Id: Ic80dd29dad4ffd1c84c2590e3d5d0e588ab2e6c2
Reviewed-on: https://go-review.googlesource.com/29051
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup_plan9.go

@@ -170,7 +170,7 @@ func lookupPort(ctx context.Context, network, service string) (port int, err err
 	case "udp4", "udp6":
 		network = "udp"
 	}
-	lines, err := queryCS(ctx, network, "127.0.0.1", service)
+	lines, err := queryCS(ctx, network, "127.0.0.1", toLower(service))
 	if err != nil {
 		return
 	}