The debug level generally just controls verbosity of libusb for debugging libusb devices/usage. We allow the environment to set the debug level independent of the application, but the application will always override this if it explicitly sets the debug level.
Changing the environment is easy, but patching the software to change the debug level isn't necessarily easy or possible. Further, there's this write-only debug_fixed variable that would seem to imply that the debug level should be fixed, but it isn't currently used. Change the logic to use strtol() so we can detect real 0 vs. conversion failure, then honor debug_fixed in libusb_set_debug.