Added some aliases:

• b' is now an official alias for break'. It used to be an unofficial alias, but this was broken by adding the bt' alias for trace'.
• t' is now an official alias for trace'. It used to be an unofficial alias, but this was broken by adding the `thread' command.
• registers' is now an alias for show registers'. This is a hack to break the unofficial r' alias for reset'. r' really means registers' in some debuggers, so I sometimes type it accidentally and am annoyed when it resets the system. A short command shouldn't have such a large effect. Now at least res' must be typed to disambiguate reset'.