taken from Hasu codebase (https://github.com/tmk/tmk_keyboard) usb.c/.h
and usb_extra.c/.h, though these files only have the PJRC
copyright/license in the header
- linked lists need to be rewritten to be more memory efficient
- all kbfun functions are now of type `(void kbfun_...(void))`, and the
arguments they need are passed via a group of global `main_arg_...`
variables (and other `main_...` variables)
