don't log anything by default
don't log anything by default

Only log messages if the user asked for them using usb_set_debug()

Ludovic Rousseau pointed out that library users may close stdout/stderr
and those file descriptors may be reused for other things.

BuLogics/libusb-compat-0.1,ystk/tools-yocto1-libusb-compat-0.1,BuLogics/libusb-compat-0.1,ystk/tools-yocto1-libusb-compat-0.1,hjelmn/libusb-compat-0.1,hjelmn/libusb-compat-0.1,ystk/tools-yocto1-libusb-compat-0.1,Distrotech/libusb-compat,Distrotech/libusb-compat,BuLogics/libusb-compat-0.1