tools/lib: atusb can now be selected by bus/device or by bus-port path

Examples:

  usb:1/6	select logical device 6 on bus 1
  usb:1-3.1.4	starting at bus 1, choose ports 3, 1, and 4

- usbopen.c (initialize, open_usb): moved libusb initialization to
  separate function, to allow for sharing
- usbopen.h (restrict_usb_path), usbopen.c (restricted_path, open_usb,
  restrict_usb_dev, restrict_usb_by_dev, read_num, restrict_usb_by_port,
  restrict_usb_path): added mechanism to restrict USB path
- atusb.c (atusb_open): if an argument is given, call restrict_usb_path
  with it

tools/lib/atusb.c

@@ -54,6 +54,8 @@ static void *atusb_open(const char *arg)
 {
 	usb_dev_handle *dev;
 
+	if (arg)
+		restrict_usb_path(arg);
 	dev = open_usb(USB_VENDOR, USB_PRODUCT);
 	if (dev) {
 		error = 0;