Two new attributes, "extension" and "devtype" now allow user-space to read
the extension type and device type. As device detection is asynchronous,
we send a CHANGED event after it is done. This also allows user-space to
wait for a device to settle before opening its input event devices.

The "extension" device is compatible with the old "extension" sysfs field
(which was registered by the static extension support code).
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>