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/config.go

@@ -0,0 +1,67 @@
+// 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 has types and a parser for representing WireGuard config.
+package wgcfg
+
+import (
+	"inet.af/netaddr"
+)
+
+// Config is a WireGuard configuration.
+// It only supports the set of things Tailscale uses.
+type Config struct {
+	Name       string
+	PrivateKey PrivateKey
+	Addresses  []netaddr.IPPrefix
+	ListenPort uint16
+	MTU        uint16
+	DNS        []netaddr.IP
+	Peers      []Peer
+}
+
+type Peer struct {
+	PublicKey           Key
+	AllowedIPs          []netaddr.IPPrefix
+	Endpoints           string // comma-separated host/port pairs: "1.2.3.4:56,[::]:80"
+	PersistentKeepalive uint16
+}
+
+// Copy makes a deep copy of Config.
+// The result aliases no memory with the original.
+func (cfg Config) Copy() Config {
+	res := cfg
+	if res.Addresses != nil {
+		res.Addresses = append([]netaddr.IPPrefix{}, res.Addresses...)
+	}
+	if res.DNS != nil {
+		res.DNS = append([]netaddr.IP{}, res.DNS...)
+	}
+	peers := make([]Peer, 0, len(res.Peers))
+	for _, peer := range res.Peers {
+		peers = append(peers, peer.Copy())
+	}
+	res.Peers = peers
+	return res
+}
+
+// Copy makes a deep copy of Peer.
+// The result aliases no memory with the original.
+func (peer Peer) Copy() Peer {
+	res := peer
+	if res.AllowedIPs != nil {
+		res.AllowedIPs = append([]netaddr.IPPrefix{}, res.AllowedIPs...)
+	}
+	return res
+}
+
+// PeerWithKey returns the Peer with key k and reports whether it was found.
+func (config Config) PeerWithKey(k Key) (Peer, bool) {
+	for _, p := range config.Peers {
+		if p.PublicKey == k {
+			return p, true
+		}
+	}
+	return Peer{}, false
+}