Update ourself and any of our subcommands.

def prog(self, prog):
        """ Update ourself and any of our subcommands. """
        self.argparser.prog = prog
        fmt = '%s %%s' % prog if prog else '%s'
        for command in self.subcommands.values():
            command.prog = fmt % command.name
        # Rebind autoenv vars with recomputed key.
        for action in self._autoenv_actions:
            self.argparser.unbind_env(action)
            self.argparser.bind_env(action, self._make_autoenv(action))