libusb.c: consolidate USB-related addvar() calls
Remove redundant usb_set_altinterface(), unless user requests it

Adds flag/value to USB driver options.

Closes networkupstools/nut#138
nut-usbinfo: ignore *.orig files
Merge branch 'issue_138_altinterface_value'

Removing the usb_set_altinterface() call seems to be a net positive, but there
is a new `usb_set_altinterface` driver option to restore the old behavior.