Thread status

[giampaolo]

This idea was inspired by https://github.com/benfred/py-spy#how-do-you-detect-if-a-thread-is-idle-or-not.
It should be possible to determine thread status:

• on Linux, by reading /proc/pid/status
• on macOS, via thread_basic_info syscall
• on Windows, by using multiple syscalls (see code)

The general idea is to be able to identify threads which are "idle", because it's useful. In order to achieve this the namedtuples returned by Process.threads() may provide either an is_active boolean field or a status field returning a string similar to the process STATUS_* constants.