IPV6_ONLY can be turned off. It is supposed to be off by default, and
  it actually is. Unfortunately, OpenBSD and NetBSD people think
  network software programmer like us are too stupid to use this
  feature properly, so they just removed it. Lets say the most stupid
  are not always the ones one think. Also, Windows doesn't support this
  yet. On all these OSes, keep the old behavior; that is prefer IPv4.
  Closes #166.

- Windows protection level compile fix