func NewFlag(flagName, defaultValue string, serverType string) *Addr {
	addr := &Addr{
		s: defaultValue,
	}
	flag.Var(addr, flagName, Usage(serverType))
	return addr
}
NewFlag returns a flag that implements the flag.Value interface.
package listen
import (
	"errors"
	"flag"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
)