libusb: hotplug, use events instead of a timer when possible

Try to fetch events from nlsysevent or devd to determine when
to scan the usb bus for devices addition or removal.
if none are available fallback on the regular timer based (4s)
scanner

if devd socket or netlink socket is closed or error fallback on the
timer based method.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D48300