linux: Cache the entire descriptors at device-init

linux: Cache the entire descriptors at device-init

Now that the core caches device-descriptors, we end up opening each
usbfs-node or sysfs-descriptors file once on libusb_init anyways.

So we might as well do this on device-init, rather then waiting for the
core to call op_get_device_descriptor. This allows us to simplify the
code in various places.

While we've it open, read the entire file rather then only reading the
device-descriptor. This is practically free, since most of the cost is in
the opening of the file, not in reading it.

Running the stress test, which does 10000 libusb_init calls, takes 21.8 seconds
on avarage on my idle system with 17 usb devices both before and after this
patch, showing that the cost of also reading the config descriptors while
we've the file open is truely neglible.

Note that this patch does not yet use the cached config descriptors, this is
done by a later patch in this series.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>