* Issue #159: Fix windows_utils.socketpair()

  - Use "127.0.0.1" (IPv4) or "::1" (IPv6) host instead of "localhost", because
    "localhost" may be a different IP address
  - Reject also invalid arguments: only AF_INET/AF_INET6 with SOCK_STREAM (and
    proto=0) are supported

* Reject add/remove reader/writer when event loop is closed.
* Fix ResourceWarning warnings