The "schedule_post_signal_hook" method of the Genode::Entrypoint class
is problematic because the signal hook can be scheduled and replaced
multiple times during the signal dispatch cycle. Add an alternative to
this method with "register_io_progress_handler" and the "Post_signal_
hook" class with "Io_progress_handler". The difference being an
"Io_progress_handler" may be registered once during the lifetime of an
entrypoint to prevent arbitrary libraries from replacing a pending hook.
The "register_io_progress_handler" remains as a deprecated API, and is
now invoked for every I/O signal received and only for I/O signals
rather than for any signal.
Ref #3132
57fd4e9148