Disable docker system dial-stdio on Windows

The `conn` here is `*winio.win32MessageBytePipe` which does not have a
`CloseRead` method (it does have `CloseWrite`) resulting in:

    docker@WIN-NUC0 C:\Users\docker>.\docker-windows-amd64.exe system dial-stdio
    the raw stream connection does not implement halfCloser

Also disable the path which uses this for cli-plugins on Windows.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli/command/system/cmd.go

@@ -1,6 +1,8 @@
 package system
 
 import (
+	"runtime"
+
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/spf13/cobra"
@@ -19,8 +21,12 @@ func NewSystemCommand(dockerCli command.Cli) *cobra.Command {
 		NewInfoCommand(dockerCli),
 		newDiskUsageCommand(dockerCli),
 		newPruneCommand(dockerCli),
-		newDialStdioCommand(dockerCli),
 	)
+	if runtime.GOOS != "windows" {
+		cmd.AddCommand(
+			newDialStdioCommand(dockerCli),
+		)
+	}
 
 	return cmd
 }