core/sys/rtimer.h, rtimer.c: support setting rtimers from interrupts (draft)

Underlying interrupt model:
- non-interrupt code can be interrupted by a timer interrupt or any
  other interrupt,
- a timer interrupt can be interrupted by a non-timer interrupt,
- non-timer interrupts cannot be interrupted by something else,
- any interrupt can call rtimer_set and rtimer_cancel,
- timer interrupts can also call rtimer_run_next.

Code is only lightly tested. May still harbour bugs.