Made cyborg middle-clicks cycle through the list of selected modules, and added some new helper procs for selected modules.

New procs:
* module_selected(module) - Checks whether the module slot specified by "module" is currently selected.
* module_active(module) - Checks whether there is a module active in the slot specified by "module".
* get_selected_module() - Returns the slot number of the currently selected module.  Returns 0 if no modules are selected.
* select_module(module) - Selects the module slot specified by "module"
* deselect_module(module) - Deselects the module slot specified by "module"
* toggle_module(module) - Toggles the selection of the module slot specified by "module".
* cycle_modules() - Cycles through the list of selected modules.

Cyborg middleclick and swap_hands verb call cycle_modules, while clicking on the UI slots calls toggle_module.

Signed-off-by: Mloc-Argent <colmohici@gmail.com>