core/sys/rtimer.c (run_deferred): avoid set_queue race; avoid partial copy

This change fixes two issues:

1) we can only test whether set_queue contains any pending rtimers when
   "setting" is set,
2) we must copy the new rtimer setting before locking (the rtimer list)
   because these settings can be changed while locked.
core/sys/rtimer.c, rtimer.h: have two queues for rtimers waiting to be set

This way we can keep different levels of interrupts out of each other's
hair.
core/sys/rtimer.c (rtimer_set): make copying of setting atomic via generation count