backend_modify
backend_modify (EV_A_ fd, anfd->events, 0);
backend_modify (EV_A_ fd, o_events, anfd->events);
backend_modify = poll_modify;
backend_modify = select_modify;
VAR (backend_modify, void (*backend_modify)(EV_P_ int fd, int oev, int nev))
#define backend_modify ((loop)->backend_modify)