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/magicsock/legacy.go

@@ -19,7 +19,6 @@ import (
 
 	"github.com/tailscale/wireguard-go/conn"
 	"github.com/tailscale/wireguard-go/tai64n"
-	"github.com/tailscale/wireguard-go/wgcfg"
 	"golang.org/x/crypto/blake2s"
 	"golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/crypto/poly1305"
@@ -28,6 +27,7 @@ import (
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/wgkey"
+	"tailscale.com/wgengine/wgcfg"
 )
 
 var (