envflag.go
func Int64(name string, value int64, usage string) *int64 {
	return EnvironmentFlags.Int64(name, value, usage)
}
func Int64(name string, value int64, usage string) *int64 {
	return EnvironmentFlags.Int64(name, value, usage)
}
// Int64 defines an int64 flag with specified name, default value, and
// usage string.  The return value is the address of an int64 variable
// that stores the value of the flag.