Remove mode_setup and mode_setup_exc callbacks from mode.

mode_setup wasn't used, and the initialization done as in mode_setup_exc
is now done after defaults have been set and arguments processed in
mode_cmd (soon "init").
mode: rename mode_cmd callback to init.

This is called when the mode is activated, and should do the following:
- set defaults on all mode-specific internal parameters
- process any tokenline arguments given
- initialize the hardware
mode: rename mode_cleanup callback to cleanup.
mode: Remove obsolete callbacks.
mode: Rename mode_cmd_exec to exec.
mode: Move "show" command to modules, and remove obsolete callback.
mode: Rename mode_str_prompt to get_prompt.
Rename all start/stop/read/write commands (-mode_).
Code cleanup.
Update tokenline from remote repository.
mode: Fix off-by-one in write:repeat syntax.
mode: Fix data count on write.
mode: Properly bomb out on invalid data on write.