scan-registers: determine available notifications

00 is documented in HID++ 1.0 specification, it should be safe to set the 00
register. As another program may have written the notifications register
already and since enabling notifications has side-effects such as "disabling"
certaing functions, restore the flags after reading out available notifs.
Merge pull request #41 from Lekensteyn/tool-updates

scan-registers: determine available notifications