Add DialOpt. So we have:

func Dial(net, addr string) (Conn, error)
func DialTimeout(net, addr string, timeout time.Duration) (Conn, error)
func DialOpt(addr string, opts ...DialOption) (Conn, error)

DialTimeout (and Dial) are regrettable in retrospect. Maybe
in a future Go we'll be back down to one Dial, with DialOpt
becoming Dial.

DialOpt looks like:

c, err := net.DialOpt("google.com:80")  // tcp is default
c, err := net.DialOpt("google.com:80", net.Timeout(30 * time.Second))
c, err := net.DialOpt("google.com:80", net.TCPFastOpen())
c, err := net.DialOpt("google.com:80", net.LocalAddr(..))
c, err := net.DialOpt("google.com:53", net.Network("udp6"))

And then: (clustered in godoc)

type DialOption interface { /* private only */ }
  func Deadline(time.Time) DialOption
  func LocalAddr(Addr) DialOption
  func Network(string) DialOption
  func TCPFastOpen() DialOption
  func Timeout(time.Duration) DialOption

I'm pretty confident we could add Happy Eyeballs to this too.

Fixes #3097
Update #3610
Update #4842

R=golang-dev, r, dave, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7365049