Avoid crashes upon C-g in nested invocations of 'read_char'

Avoid crashes upon C-g in nested invocations of 'read_char'

* src/keyboard.c (read_char, read_event_from_main_queue):
Ensure the global value of getcjmp is restored when the stack
is unwound by the likes of 'throw', by calling
record_unwind_protect_ptr instead of restoring the value
manually.  (Bug#34394)
(restore_getcjmp): Argument is now 'void *', to match the
signature of record_unwind_protect_ptr.