sched_algorithm_with_properties<> must intercept awakened() calls before
forwarding control to the subclass override. That pretty much requires two
different virtual methods: one for sched_algorithm subclasses WITHOUT
properties, the other for sched_algorithm_with_properties subclasses.
Originally we used a different name (awakened_props()), but that was
bothersome. It makes more sense to use a different awakened() overload
instead, one that passes in the relevant properties object.
Fix examples/priority.cpp accordingly.
Also delegate instantiation of a new properties object to new_properties()
virtual method. Overriding this method allows you to customize allocation,
instead of sched_algorithm_with_properties<> unconditionally putting the new
properties object on the heap. Validate the new_properties() return value.
