Input: keyboard - fix lack of locking when traversing handler->h_list

Keyboard handler should not attempt to traverse handler->h_list on
its own, without any locking, otherwise it races with registering
and unregistering of input handles which leads to crashes.

Introduce input_handler_for_each_handle() helper that allows safely
iterate over all handles attached to a particular handler and switch
keyboard handler to use it.

Reported-by: Jim Paradis <jparadis@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>