This CL provides IPv6 scoped addressing zone support as defined
in RFC 4007 for internet protocol family connection setups.

Follwoing types and functions allow a literal IPv6 address with
zone identifer as theirs parameter.

pkg net, func Dial(string, string) (Conn, error)
pkg net, func DialOpt(string, ...DialOption) (Conn, error)
pkg net, func DialTimeout(string, string, time.Duration) (Conn, error)
pkg net, func Listen(string, string) (Listener, error)
pkg net, func ListenPacket(string, string) (PacketConn, error)
pkg net, func ResolveIPAddr(string, string) (*IPAddr, error)
pkg net, func ResolveTCPAddr(string, string) (*TCPAddr, error)
pkg net, func ResolveUDPAddr(string, string) (*UDPAddr, error)
pkg net, type IPAddr struct, Zone string
pkg net, type TCPAddr struct, Zone string
pkg net, type UDPAddr struct, Zone string

Also follwoing methods return a literal IPv6 address with zone
identifier string if possible.

pkg net, method (*IPAddr) String() string
pkg net, method (*TCPAddr) String() string
pkg net, method (*UDPAddr) String() string

Fixes #4234.
Fixes #4501.
Update #5081.

R=rsc, iant
CC=golang-dev
https://golang.org/cl/6816116