libusb: implement libusb_pollfds_handle_timeouts

This function tells applications who maintain the pollfds themselves if
they should handle the timeout for each xfer themselves. In FreeBSD, the
timeout for each xfer is handled by kernel and doesn't need a special timer to
do so. Therefore, we return 1 to indicate that it is handled by libusb
internally.

Approved by: lwhsu (mentor), markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51742