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The commentate of function SockaddrToIPAndZone is same with
IPAndZoneToSockaddr. Repaired.
Signed-off-by: gukq <gukaiqiang@gmail.com>

1. Never panic.
2. Only return a unspecified address if the IP is nil (no other random error
cases).
3. Simplify the logic.

They're valid.

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

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.

syscall is frozen and mostly deprecated.