envflag.go
func Int64Var(p *int64, name string, value int64, usage string) {
	EnvironmentFlags.Int64Var(p, name, value, usage)
}
func Int64Var(p *int64, name string, value int64, usage string) {
	EnvironmentFlags.Int64Var(p, name, value, usage)
}
// Int64Var defines an int64 flag with specified name, default value,
// and usage string.  The argument p points to an int64 variable in
// which to store the value of the flag.