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set the V4InV6Prefix correctly (like go does)

This is why go uses 16 byte arrays for all addresses by default.

fix windows support

1. Switch back to the syscall package for errors. That way, we get consistent
   errors on all platforms.
2. Use `sys/windows` where appropriate.
3. Export type aliases for the *Sockaddr* types. That way, we can use the
   platform-specific types but call them the same name, regardless of platform.

switch to golang.org/x/sys

syscall is frozen and mostly deprecated.