cluster: fix libuv assert in net.listen()

Problem: calling `server.listen()` (no port) on a net.Server triggered the
following libuv assertion:

  node: ../deps/uv/src/unix/stream.c:406: uv__write: Assertion `fd_to_send >= 0'
  failed.

Cause: uv_tcp_t handles are lazily initialized. Omitting the port made the
handle get initialized even more lazily. Too lazily - it wasn't initialized
when the handle was sent over to the child process.

Solution: implicitly bind to a random port in listen() when the port number
is omitted, it forces the handle to initialize. This is not a change in
behavior, listen() has always been identical to listen(0).

Fixes #3325.