libusb: implement libusb_free_pollfds

libusb provides an API that frees the storage for pollfds regardless of
their contents. It does not check whether the file descriptors inside
are already closed.

Approved by: markj (mentor), lwhsu(mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51221