Flags: add user-defined flag types. The change is really no code; it's just publishing

Flags: add user-defined flag types. The change is really no code; it's just publishing
the set() method and add() functions.  But we rename add() to Var() for consistency.
Also rename FlagValue to Value for simplicity.

Also, delete the check for multiple settings for a flag.  This makes it possible to
define a flag that collects values, such as into a slice of strings.

type flagVar []string

func (f *flagVar) String() string {
	return fmt.Sprint(v)
}

func (f *flagVar) Set(value string) bool {
	if v == nil {
		v = make(flagVar, 1)
	} else {
		nv := make(flagVar, len(v)+1)
		copy(nv, v)
		v = nv
	}
	v[len(v)-1] = value
	return true
}

var v flagVar

func main() {
	flag.Var(&v, "testV", "multiple values build []string")
	flag.Parse()
	fmt.Printf("v = %v\n", v)
}

R=rsc
CC=golang-dev
https://golang.org/cl/842041