Add new evsignal_add_with_flags() for signal events

This patch, change `struct event`, and this can break binary compatibility.
If this is a major issue, we can fix this patch to avoid breaking it.
Thanks.
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By default libevent set SA_RESTART flag for sigaction(2), which will restart
syscall automatically, and this is not always good.
(That because it need to be compitable with signal(2), while it vary
across systems)

This patch add new function evsignal_add_with_flags(), which will change
this flag from default SA_RESTART to user defined.