Support LIBUSB_DEBUG=NUM setting in ups.conf

[jimklimov]

Addresses part of #2616

A quick hack for that issue, to setenv() the envvar during configuration parsing, so hopefully the library would see it:

:; ./drivers/usbhid-ups -a eco650 -x LIBUSB_DEBUG=qqq -d 1
Network UPS Tools 2.8.2.1082.2-1084-g0520091ac (development iteration after 2.8.2) - Generic HID driver 0.57
USB communication driver (libusb 1.0) 0.49
WARNING : Invalid LIBUSB_DEBUG value found in ups.conf for the driver, defaulting to 4
Enabling LIBUSB_DEBUG=4 from ups.conf
[timestamp] [threadID] facility level [function call] <message>
--------------------------------------------------------------------------------
[ 0.000011] [001a9956] libusb: debug [libusb_init] created default context
[ 0.000019] [001a9956] libusb: debug [libusb_init] libusb v1.0.24.11584
...
ups.timer.shutdown: -1
ups.timer.start: -1
ups.vendorid: 0463
[ 7.381093] [001aa8d0] libusb: debug [libusb_close]
[ 7.381098] [001aa8d0] libusb: debug [usbi_remove_event_source] remove fd 8
[ 7.381120] [001aa8d0] libusb: debug [libusb_exit]
[ 7.381123] [001aa8d0] libusb: debug [libusb_exit] destroying default context
[ 7.381126] [001aa8d0] libusb: debug [libusb_handle_events_timeout_completed] doing our own event handling
[ 7.381129] [001aa8d0] libusb: debug [handle_events] event sources modified, reallocating event data
[ 7.381134] [001aa8d0] libusb: debug [usbi_wait_for_events] poll() 2 fds with timeout in 0ms
[ 7.381137] [001aa8d0] libusb: debug [usbi_wait_for_events] poll() returned 0
[ 7.381140] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 6.1
[ 7.381143] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 5.1
[ 7.381146] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 4.1
[ 7.381149] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 3.7
[ 7.381153] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 3.6
[ 7.381157] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 3.1
[ 7.381160] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 2.1
[ 7.381165] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 1.4
[ 7.381167] [001aa8d0] libusb: debug [libusb_unref_device] destroy device 1.1
[ 7.381170] [001aa8d0] libusb: debug [usbi_remove_event_source] remove fd 7
[ 7.381177] [001aa8d0] libusb: debug [usbi_remove_event_source] remove fd 6
[ 7.381198] [001aa8d1] libusb: debug [linux_udev_event_thread_main] udev event thread exiting

A cleaner solution would be to call libusb_set_debug() specifically, when initializing a context for device communications, but needs some more careful coding :) so can be a follow-up sometime later.

FYI : CC @masterwishx @tormodvolden