add soreuseport option of socket, this feature come from linux 3.9
(http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=055dc21a1d1d219608cd4baac7d0683fb2cbbe8a).
you can set  "reuse_port on" in event block to open soreuseport option.
Merge pull request #509 from monadbobo/reuseport_fixed

add soreuseport option of socket.