Possible loop in caml_enter_blocking_section when no domain can handle a blocked signal

[abbysmal]

This can be triggered when a domain that blocked a specific set of signals ends up in a situation where no other domain can process the signal (for example: it is the last domain and a signal was recorded by another domain beforehand).

What can happen is that caml_enter_blocking_section will trigger a loop because processing all signals is made impossible:
This domain cannot possibly process the signal because the logic in caml_process_pending_signals makes it so that since it is masked, it will leave it unprocessed.

In normal circumstances, another domain that does not block it will eventually process the signals in question and this domain will eventually be freed, but when no other domain can do this, we end up in a loop.