Add usable_cpu_count() function

[giampaolo]

Recently os.cpu_count() on Windows has been fixed in order to take process groups into account and return the number of all available CPUs:
http://bugs.python.org/issue30581
The fix will be included in psutil too (#771) but that's another story.

This made me realize that psutil.cpu_count() does not return the number of usable CPUs, which could possibly represent a better default value for things like multiprocessing and process pools.
It is currently possible to retrieve this info on UNIX with len(os.sched_getaffinity(0)) which takes CPU affinity and (I think) Linux cgroups into account, but it's not possible to do the same on Windows which is supposed to provide this value via GetActiveProcessorCount() API.

[giampaolo]

psutil.cpu_usable_count() would probably be a better name as right now all system functions with the exception of memory-related ones use a prefix which makes sense (cpu_*, disk_*, net_*, etc.).

[giampaolo]

Actually I'm confused: I now realize it should already be possible to get usable CPU count on Windows with len(psutil.Process().cpu_affinity()). Internally it uses GetProcessAffinityMask but I'm not sure how it differs from GetActiveProcessorCount().

[KOLANICH]

On Linux the both ways return the number of currently enabled cpu cores.
The correct value can be obtained from the files in /sys/devices/system/cpu/

Also these files can be used to brign up/down cores and check their statuses.

[tiran]

It's actually more complicated. cgroups can limit the amount of usable CPUs, see https://bugs.python.org/issue36054

[dhirschfeld]

Windows Containers use Job objects to limit the number of CPUs so to work inside a container would require querying this object