This runs through events pending at init on initialization so we can
tell if some devices are available.
Note that with the way wlr_device_lists is managed, this checks that
there is at least one device we handle - it doesn't have to be a
keyboard, but there is at least a mouse or tablet_pad or something
that we care about.
Instead of failing inconditionally it might be better to leave the
decision to the user, e.g. add a "backend_has_devices" function to
call later.
(Tested by moving /dev/input off)
Fixes#24.
880c239657