Related issues: 4501, 4234

The package net contains some APIs that fetches the kernel state, specifically
network interfaces and its belongings for creating new connections. issue #4501,
4234 focus on transport API, Dial and Listen, but are not for under the hood API.

This issue holds that how could we support IPv6 scoped address zone support 
for such under the hood API, including whether it should be a part of net API.

API that would be affected:
type IPNet struct
method (*IPNet) String
type IPAddr struct
method (*IPAddr) String
func Interfaces
func InterfaceAddrs
method (*Interface) Addrs
method (*Interface) MulticastAddrs