Allow -1 as valid fd in libevdev_change_fd

Allow -1 as valid fd in libevdev_change_fd

Add a new flag for "initialized" and separate that from the fd logic. This way,
we can call libevdev_change_fd(dev, -1) to signal that the current fd should be
dropped.

Otherwise libevdev can't be told to release the fd and always keeps a reference
to it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>