Implement UNIXServer and UNIXSocket.for_fd

[headius]

Since UNIXServer and UNIXSocket use a real fd and jnr-enxio, they could both support for_fd without too much effort.

One minor issue that would need to be fixed is that there's no way to create a UNIXSocket from jnr/jnr-unixsocket with a given fd; it handles creating the fd in its factory methods and does not expose the fd-receiving constructor.

[kares]

done at #3494