Fix signal handler

[SF Markus Elfring]

>> 2. Would it eventually need to be protected by a mutex (from the
>>    multi-threading perspective)?
> 
> Using a mutex in a signal handler is neither conforming nor working in
> practise, so far worse than the current approach.

I find it interesting that the statements "ECB_MEMORY_FENCE_ACQUIRE" and
"ECB_MEMORY_FENCE_RELEASE" were added recently.
http://cvs.schmorp.de/libev/ev.c?r1=1.448&r2=1.449

Regards,
Markus