Handle hierarchy events for updating scroll valuators correctly

Handle hierarchy events for updating scroll valuators correctly

* src/keyboard.c (gen_help_event, kbd_buffer_store_help_event):
Make sure to initialize the event buffer correctly.

* src/xterm.c (xi_populate_device_from_info): New function.
(x_init_master_valuators): Factor out most of the valuator
tracking code to that function.
(handle_one_xevent): Handle device enable and disable events in
a more detailed fashion.