Print password prompts to stderr

This makes rclone with encrypted config better suited for use in
pipelines. E.g.:

$ rclone lsl mydrive:Some/Dir | sort -k 4

If the password prompt ("Enter configuration password") is printed to
stdout, it will be swallowed by sort. By printing it to stderr, you
still see the prompt, without sacrificing compatibility with the unix
pipeline.

fs/config_read_password.go

@@ -18,7 +18,7 @@ import (
 // ReadPassword reads a password without echoing it to the terminal.
 func ReadPassword() string {
 	line, err := terminal.ReadPassword(int(os.Stdin.Fd()))
-	fmt.Println("")
+	fmt.Fprintln(os.Stderr)
 	if err != nil {
 		log.Fatalf("Failed to read password: %v", err)
 	}