[GAP] Deferred task aborting and get_immediate

[Arnaud Charlet]

> I believe GNARL did support abort of threads blocked on input,
> when I worked on it.  That is, if you send an abort signal
> to a thread blocked on I/O, and you have opened the file with
> in most Unix/Linux systems any pending read operation should return with
> E_INTR and allow GNARL to check for pending abort (before restarting the
> read, if necessary).  Has this changed?

As I said, it's highly dependent on the OS behavior, so it may or may
not work and it's not a good idea to rely on this capability, hence my
suggestions.

Also what has changed is in the context of the ZCX run-time, where
async abort cannot be implemented in a reliable way, so one has to use
the sjlj run-time (--RTS=sjlj).

There are also OSes (e.g. Windows) which does not support async abort
at all.

Arno