libusb: improve compatibility

Specifically, add LIBUSB_CLASS_PHYSICAL and the libusb_has_capability API.
Descriptions and functionality for these derived from the
documentation at [0]. The current set of capabilities are all supported by
libusb.

These were detected as missing after updating net/freerdp to 2.1.1, which
attempted to use both.

[0] http://libusb.sourceforge.net/api-1.0/group__libusb__misc.html

Reviewed by: hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25194