Implement Remote Control for rclone #2111

This implements a remote control protocol activated with the --rc flag
and a new command `rclone rc` to use that interface.

Still to do
  * docs - need finishing
  * tests

fs/rc/internal.go

@@ -0,0 +1,50 @@
+// Define the internal rc functions
+
+package rc
+
+import "github.com/pkg/errors"
+
+func init() {
+	Add(Call{
+		Path:  "rc/noop",
+		Fn:    rcNoop,
+		Title: "Echo the input to the output parameters",
+		Help: `
+This echoes the input parameters to the output parameters for testing
+purposes.  It can be used to check that rclone is still alive and to
+check that parameter passing is working properly.`,
+	})
+	Add(Call{
+		Path:  "rc/error",
+		Fn:    rcError,
+		Title: "This returns an error",
+		Help: `
+This returns an error with the input as part of its error string.
+Useful for testing error handling.`,
+	})
+	Add(Call{
+		Path:  "rc/list",
+		Fn:    rcList,
+		Title: "List all the registered remote control commands",
+		Help: `
+This lists all the registered remote control commands as a JSON map in
+the commands response.`,
+	})
+}
+
+// Echo the input to the ouput parameters
+func rcNoop(in Params) (out Params, err error) {
+	return in, nil
+}
+
+// Return an error regardless
+func rcError(in Params) (out Params, err error) {
+	return nil, errors.Errorf("arbitrary error on input %+v", in)
+}
+
+// List the registered commands
+func rcList(in Params) (out Params, err error) {
+	out = make(Params)
+	out["commands"] = registry.list()
+	return out, nil
+}