Already present in all other OSes. Unfortunately, not great on
gnulinux as the clock used for timing out is the realtime clock.

This allows to specify a timeout on aperiodic activities, which
is how long the activity will wait for a trigger before being called
anyways. This is meant to implement timeout mechanisms in case
no trigger ever comes (e.g. on port-driven tasks)

Otherwise task context's hooks are not called.

…hanged