Add Null POSIX backend
    
    Add a simple null backend for POSIX platforms that reports no available
    devices, and provides no capabilities. Make use of this new backend on
    all the OSes without an existing backend, so libusb can be built even on
    OSes without USB support.
    
    (cherry picked from commit 53572d7e5eee79266139399924c7491174be1670)
    
    patch_name: 0006-Add-Null-POSIX-backend.patch
    present_in_specfile: true
    location_in_specfile: 6
    squash_commits: true