MINOR: cpuset: centralize a reliable bound cpu detection

MINOR: cpuset: centralize a reliable bound cpu detection

Till now the CPUs that were bound were only retrieved in
thread_cpus_enabled() in order to count the number of CPUs allowed,
and it relied on arch-specific code.

Let's slightly arrange this into ha_cpuset_detect_bound() that
reuses the ha_cpuset struct and the accompanying code. This makes
the code much clearer without having to carry along some arch-specific
stuff out of this area.

Note that the macos-specific code used in thread.c to only count
online CPUs but not retrieve a mask, so for now we can't infer
anything from it and can't implement it.

In addition and more importantly, this function is reliable in that
it will only return a value when the detection is accurate, and will
not return incomplete sets on operating systems where we don't have
an exact list, such as online CPUs.