refactor redundant code around calls to cmd.Parse

Signed-off-by: Tibor Vass <teabee89@gmail.com>

utils/flags.go

@@ -0,0 +1,34 @@
+package utils
+
+import (
+	"fmt"
+	"os"
+
+	flag "github.com/docker/docker/pkg/mflag"
+)
+
+// ParseFlags is a utility function that adds a help flag if withHelp is true,
+// calls cmd.Parse(args) and prints a relevant error message if there are incorrect number of arguments.
+// TODO: move this to a better package than utils
+func ParseFlags(cmd *flag.FlagSet, args []string, withHelp bool) error {
+	var help *bool
+	if withHelp {
+		help = cmd.Bool([]string{"#help", "-help"}, false, "Print usage")
+	}
+	if err := cmd.Parse(args); err != nil {
+		return err
+	}
+	if help != nil && *help {
+		cmd.Usage()
+		// just in case Usage does not exit
+		os.Exit(0)
+	}
+	if str := cmd.CheckArgs(); str != "" {
+		if withHelp {
+			str += ". See 'docker " + cmd.Name() + " --help'"
+		}
+		fmt.Fprintf(cmd.Out(), "docker: %s.\n", str)
+		os.Exit(1)
+	}
+	return nil
+}