libusb no longer caches descriptors in libusb_device but backends are
intended to be able to provide copies from memory. In the common linux
case we can use sysfs.
Devices are now assigned a session ID (currently busnum:devaddr) which
is used to distinguish unique devices.
Now multiple callers of libusb_get_device_list will get the same
libusb_device structure instances.
libusb_find_devices and libusb_get_devices are no more
libusb_get_device_list obtains a list of libusb_device structures for all
known devices in the system.
Each libusb_device now has a reference count, defaulting to 1 on
instantiation. The reference count of 1 refers to the fact that it is
present in the list in this scenario.
Opening a device adds a pointer to the libusb_device structure in the
handle, so that also adds a reference. Closing the device removes that
reference.
The function to free the device list can optionally unref all the devices
inside.
In future we will make the libusb_device instances all "global" so that if
the app calls get_device_list twice it actually gets the same libusb_device
structure references back. This way we can start to track disconnects, and
we can investigate adding a unique "session ID" to each libusb_device, an
identifier guaranteed to be unique to that device until reboot.
Instead of timers, add a mechanism for informing the parent app when the
next timeout is due to happen, so that it can call us at that time.
As we no longer use signals, signalfd has also been removed.
fpi changed to usbi.
We should not expose structures with prefix "usb_" in the public
namespace as it is quite likely there will be some conflict somewhere.
Instead, using "libusb_" should be safer.
