core/sys/rtimer.c: poll if outcome of rtimer_arch_schedule is uncertain

If the expiration time of an rtimer is in the past, present, or very near
future, rtimer_arch_schedule may not schedule it properly. We therefore
poll the queue "manually" if the next timer may have expired before or
during the call to rtimer_arch_schedule.
core/sys/rtimer.[ch]: whitespace changes to follow Contiki's ugly coding standard
core/sys/rtimer.c: further uglification (adding redundant braces)