Presently, commands registration taks a static handler data pointer.
This patch adds support for commands that require a dynamic pointer,
such as those registered in a dynamic context (e.g. subcommands for a
user-created 'foo.cpu' command).  The command_set_handler_data will
update a command (group) to use a new context pointer, while the
CMD_DATA macro allows command handlers to access the value.
Jim handlers should find this value in interp->cmdPrivData.