Add option to opt out of sock flag reuseaddr in socket.lua:bind

[Fish013]

I think there are valid use-cases where i want to bind to a address/port but want to fail if the address is already in use.

luasocket/src/socket.lua

Line 40 in 1d61853

sock:setoption("reuseaddr", true)

[alerque]

That sounds like a reasonable thing to need. Contributions are welcome. I'm happy to facilitate PRs but will likely not have a chance to hack on this myself for some time.