Introduce PSUTIL_DEBUG mode (via env var)

[giampaolo]

This occurred to me while debugging #1152. It would be extremely useful to be able to run psutil in DEBUG mode and print useful messages on stderr in order to debug nasty issues. In order to use it one would have to start python like this:

PSUTIL_DEBUG=1 python script.py

On Windows:

set PSUTIL_DEBUG=1 && C:\Python36\python.exe script.py

Something like this will likely be used only for error conditions, before raising an exception, and never for informational logging. Currently if a C function body has 2 syscalls, both raising OSError, the Python exception does not tell you which one failed. With this in place we can do:

if (! syscall1()) {
    debug("syscall1 failed");
    return PyErr_SetFromErrno(PyExc_OSError);
}
if (! syscall2()) {
    debug("syscall2 failed");
    return PyErr_SetFromErrno(PyExc_OSError);
}

...and the traceback will look like this:

psutil-debug> syscall2 failed
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 2] No such file or directory: 'foo'

This will make users able to submit better reports on the bug tracker. Unit tests and CI services will likely have PSUTIL_DEBUG enabled by default for the same reason.