nut-scanner: nutscan_init() called before nut_debug_level is set; library scan messages are hidden

[clepple]

nut_debug_level starts out as 0, but the -D flags are not parsed until after nutscan_init() is called. Therefore, it is not possible to see the debug messages when searching for libraries.

Tested with https://github.com/networkupstools/nut/tree/libusb-1.0%2B0.1 @ 80bc452.

Noticed while debugging #499.