[sane-devel] sanei_usb improvements

[Stef]

     Hello,

     I have pushed the sanei_usb improvements:
     - now there is a sanei_usb_exit() that frees resources allocated by 
sanei_usb_ini()t
     - instead of calling sanei_usb_init() each time we want to have an 
updated USB device list, we can call the sanei_usb_scan_devices() function

     The typical workflow of a backend using USB is now:
     - call sanei_usb_init() in sane_init()
     - call sanei_usb_scan_devices() each time sane_get_devices() is 
called to have an up-to-date list of USB scanners
     - call sanei_usb_exit() in sane_exit() to free resources

     There are now several test programs in testsuite/sanei to test 
sanei_* functions. They are run through the 'check' target (e.g. 'make 
check'). The test_wire.c test program has been moved from sanei to this 
new sub directory.

Regards,
     Stef