all: move wgcfg from wireguard-go

This is mostly code movement from the wireguard-go repo.

Most of the new wgcfg package corresponds to the wireguard-go wgcfg package.

wgengine/wgcfg/device{_test}.go was device/config{_test}.go.
There were substantive but simple changes to device_test.go to remove
internal package device references.

The API of device.Config (now wgcfg.DeviceConfig) grew an error return;
we previously logged the error and threw it away.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/wgcfg/device.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2021 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 wgcfg
+
+import (
+	"io"
+	"sort"
+
+	"github.com/tailscale/wireguard-go/device"
+	"tailscale.com/types/logger"
+)
+
+func DeviceConfig(d *device.Device) (*Config, error) {
+	r, w := io.Pipe()
+	errc := make(chan error, 1)
+	go func() {
+		errc <- d.IpcGetOperation(w)
+		w.Close()
+	}()
+	cfg, err := FromUAPI(r)
+	if err != nil {
+		return nil, err
+	}
+	if err := <-errc; err != nil {
+		return nil, err
+	}
+
+	sort.Slice(cfg.Peers, func(i, j int) bool {
+		return cfg.Peers[i].PublicKey.LessThan(&cfg.Peers[j].PublicKey)
+	})
+	return cfg, nil
+}
+
+// ReconfigDevice replaces the existing device configuration with cfg.
+func ReconfigDevice(d *device.Device, cfg *Config, logf logger.Logf) (err error) {
+	defer func() {
+		if err != nil {
+			logf("wgcfg.Reconfig failed: %v", err)
+		}
+	}()
+
+	prev, err := DeviceConfig(d)
+	if err != nil {
+		return err
+	}
+
+	r, w := io.Pipe()
+	errc := make(chan error)
+	go func() {
+		errc <- d.IpcSetOperation(r)
+	}()
+
+	err = cfg.ToUAPI(w, prev)
+	if err != nil {
+		return err
+	}
+	w.Close()
+	return <-errc
+}