curses: Use RARROW to mark selected item
module: Support auto enabling and enforce single registration of module

Enforces a single registration of a module with a subsystem.
module_register() now returns -EBUSY if module is already
registered.

Adds support for BMON_MODULE_AUTO to flag modules that should
be autoloaded upon the first call to module_set(). Loading
will succeed if the probe() function succeeds.