FreeBSD support for usb-connected devices in adb

FreeBSD (and other systems as well) may use libusb to work with USB devices.
libusb is integrated in FreeBSD base system in recent builds (8.0+),
however in other systems it may need include libusb.h like <libusb/libusb.h>
and install library from devel/libusb port or other repository.

Submitted change not supports events (attach/detach) as libusb 1.0 lacks
such functionality and in order to make code more portable I've not used
workarounds as in earlier changeset.

Code was tested on FreeBSD 8.0-RC1, HTC Hero (1.0.0.A6288, Android 1.5)