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/watchdog.go

@@ -12,7 +12,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/tailscale/wireguard-go/wgcfg"
 	"inet.af/netaddr"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn/ipnstate"
@@ -21,6 +20,7 @@ import (
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/tsdns"
+	"tailscale.com/wgengine/wgcfg"
 )
 
 // NewWatchdog wraps an Engine and makes sure that all methods complete