Enforce PSUTIL_DEBUG

[giampaolo]

In #1173 we introduced PSUTIL_DEBUG environment variable which can be set in order to print more information on stderr in order to understand where in the C code an error originates from in case of exception. #1394 is an example of how this would be useful, but in that case proc_exe C function does not print any debug message. The task is to extend this paradigm to more (possibly all?) C functions/code.

[giampaolo]

Note to self: even if a bit unorthodox, a different way to approach this would be using PyErr_SetFromWindowsErrWithFilename instead, and instead of the filename str we pass a useful message, like the name of the syscall which failed.

[giampaolo]

I opted for implementing the PyErr_SetFromWindowsErrWithFilename trick on Windows only in #1428. Other platforms just raise the usual OSError but the traceback message is changed (inlcudes the syscall). Closing this out as outdated.