System.poll: Do not poll for negative values.
System.poll: Do not poll for negative values.

We were lucky, the problem was hidden by the fact that the Scheduler was over-aggressive
with the SCHED_IMMEDIATE magic value.

	* src/object/system.cc (poll): If deadline is in the past, poll for 0.

aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi