Make it possible to construct a UNIXSocket or server with existing fd

[headius]

For jruby/jruby#940, we need a factory method or constructor method exposed that can take an existing fd.

[mrbrdo]

@headius need this for spring to work... I can't invoke the fd, opts constructor from JRuby. I wasn't even able to do it with getDeclaredConstructor (maybe some derp), but I finally managed to totally hack it with getDeclaredConstructors. So I know in fact it will work (tested it), just need to expose it to JRuby and use it for UNIXSocket.for_fd. I think UNIXSocket.for_fd is returning something right now, but it's not correct as it doesn't work.

FYI this is the (terrible) hack I used for testing: https://gist.github.com/mrbrdo/e5bcd63e0688422fb69b

[headius]

I'll get something basic in today so we can release this and you can update rails/spring#449 to use it.

[headius]

jnr-unixsocket 0.10 is released. It will be incorporated into JRuby 9.0.5.0 and is already being used on JRuby master.