libusb: implement `libusb_get_parent`

[obiwac_gmail.com]

Summary

Newer versions of drivers such as libwacom (graphics tablets) or libfprint (fingerprint scanners) call g_usb_device_get_parent. This in turn calls libusb_get_parent on platforms which implement it, and returns NULL on platforms that don't. This patch implements this function on FreeBSD.

Test Plan

Tried different combinations of different devices connected to different hubs and it all seems to work. Tried on the AMD Framework 13. Used an ad-hoc program to test this but I'd eventually like to add a tree view kind of thing to usbconfig like in lsusb.

[kevans]

Implementation wise, this looks fine to me. Can you update libusb.3 as well and document this, maybe right around libusb_get_device, please?

[obiwac_gmail.com]

• libusb.3: add libusb_get_parent

[obiwac_gmail.com]

• libusb.3: add libusb_get_parent