Investigate whether and when perf_event_open() is process-wide or thread-wide

From the [man page](http://man7.org/linux/man-pages/man2/perf_event_open.2.html):

```
The pid and cpu arguments allow specifying which process and CPU to
monitor:

pid == 0 and cpu == -1
This measures the calling process/thread on any CPU.
```

So what now, is it the calling process or thread? How do I control that?

Quite likely, if I run `perf_event_open()` from a forked pthread, it's going to count only for that pthread. But what if I run it from the process's (only) main thread, and afterwards create a new pthread? Does the perf FD then record the events only for the main thread, or the whole process?

That would be important to know, so that we don't accidentally give wrong numbers when running in the threaded runtime.