--help option and help command should print to stdout not stderr

--help and help are successful commands so output should not go to error.

    QE teams have requested this change, also users doing docker help | less
    or docker run --help | less would expect this to work.

    Usage statement should only be printed when the user asks for it.
    Errors should print error message and then suggest the docker COMMAND --help
    command to see usage information.

    The current behaviour causes the user to have to search for the error message
    and sometimes scrolls right off the screen.  For example a error on a
    "docker run" command is very difficult to diagnose.

    Finally erros should always exit with a non 0 exit code, if the user
    makes a CLI error.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Upstream-commit: a2b529ead21e6ab9eafcb1b1d2437c725c43a06a
Component: engine

components/engine/api/client/cli.go

@@ -75,8 +75,8 @@ func (cli *DockerCli) Cmd(args ...string) error {
 	if len(args) > 0 {
 		method, exists := cli.getMethod(args[0])
 		if !exists {
-			fmt.Println("Error: Command not found:", args[0])
-			return cli.CmdHelp()
+			fmt.Fprintf(cli.err, "docker: '%s' is not a docker command. See 'docker --help'.\n", args[0])
+			os.Exit(1)
 		}
 		return method(args[1:]...)
 	}
@@ -90,9 +90,10 @@ func (cli *DockerCli) Subcmd(name, signature, description string) *flag.FlagSet
 		if flags.FlagCountUndeprecated() > 0 {
 			options = "[OPTIONS] "
 		}
-		fmt.Fprintf(cli.err, "\nUsage: docker %s %s%s\n\n%s\n\n", name, options, signature, description)
+		fmt.Fprintf(cli.out, "\nUsage: docker %s %s%s\n\n%s\n\n", name, options, signature, description)
+		flags.SetOutput(cli.out)
 		flags.PrintDefaults()
-		os.Exit(2)
+		os.Exit(0)
 	}
 	return flags
 }