patch: improved signal handling (for discussion)

>
> Following on from my last message, here's the first
> pass at a patch to replace signal() calls with sigaction().
> As you can tell, it's against the 3.10-4 tree.
>
> I'd appreciate any feedback.
>
> Some notes:
> - in UnixOSProcessPlugin.c, we replace signal() with
> sigaction().  This is the pattern we use.
> - in aio.c, I introduced a tiny function, aioShimHandler,
> to call forceInterruptCheck.  This was to work around a
> compiler warning (the function prototypes differed).
> It also helped with printf debugging.
> - in aio.c, it's not clear to my why SIGPIPE is ignored.
> Can anyone explain it?  I've updated this code.
> - in aio.c, aioFini never used to clear the SIGIO handler.
> Why is this?  I've done it now, with no obvious ill effects,
> but that doesn't count for a lot when dealing with signals.
> - in sqUnixMain.c, I've added a default handler for HUP,
> USR1, USR2, and TERM.  The default action for these
> signals is to terminate the VM.  This way, they just
> complain a little to stderr, unless a handler has been
> set in smalltalk.  Now, if a signal arrives before the
> smalltalk is ready, they just ignore it.  Actually, it
> would be better to block / queue signals, which would
> then be delivered when the handler is installed. Hmmm.
>
> - Andrew