Respect 'local_only' parameter of sane_get_devices()

Some backends (for example epson2, dell1600n_net, magicolor) ignore the local_only parameter of sane_get_device(). This MR tries to fix that. This issue is problematic, because colord uses sane to find local devices. This scan runs on every change in the usb and scsi subsystems. Thus the presence of this network traffic leaks information about usb/scsi changes to the local network.

I hopefully found all instances of such backends.

test_prog.c

Closes #130 (closed), #140 (closed)