cmd/tailscaled: use the standard flag page instead of getopt

Per discussion with @crawshaw. The CLI tool already used std flag anyway.
If either of them, it would've made more sense for the CLI to use getopt.

types/flagtype/flagtype.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package flagtype defines flag.Value types.
+package flagtype
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+)
+
+type portValue struct{ n *uint16 }
+
+func PortValue(dst *uint16, defaultPort uint16) flag.Value {
+	*dst = defaultPort
+	return portValue{dst}
+}
+
+func (p portValue) String() string { return fmt.Sprint(p.n) }
+func (p portValue) Set(v string) error {
+	if v == "" {
+		return errors.New("can't be the empty string")
+	}
+	if strings.Contains(v, ":") {
+		return errors.New("expecting just a port number, without a colon")
+	}
+	n, err := strconv.ParseUint(v, 10, 64) // use 64 instead of 16 to return nicer error message
+	if err != nil {
+		return fmt.Errorf("not a valid number")
+	}
+	if n > math.MaxUint16 {
+		return errors.New("out of range for port number")
+	}
+	*p.n = uint16(n)
+	return nil
+}