net: fix Dial(":80") on Windows

Windows sockets allow bind to 0.0.0.0:80 but not connect to it.
To make Listen(":80") / Dial(":80") work as documented on Windows,
connect to 127.0.0.1 or ::1 (depending on network) in place of 0.0.0.0.

Fixes #6290.

Change-Id: Ia27537067276871648546678fbe0f1b8478329fe
Reviewed-on: https://go-review.googlesource.com/32101
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/dial_test.go

@@ -55,6 +55,23 @@ func TestProhibitionaryDialArg(t *testing.T) {
 	}
 }
 
+func TestDialLocal(t *testing.T) {
+	ln, err := newLocalListener("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+	_, port, err := SplitHostPort(ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	c, err := Dial("tcp", JoinHostPort("", port))
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.Close()
+}
+
 func TestDialTimeoutFDLeak(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9":