all: gofmt with Go 1.17

This adds "//go:build" lines and tidies up existing "// +build" lines.

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

wgengine/monitor/monitor_linux.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !android
 // +build !android
 
 package monitor

wgengine/monitor/monitor_polling.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build (!linux && !freebsd && !windows && !darwin) || android
 // +build !linux,!freebsd,!windows,!darwin android
 
 package monitor

wgengine/monitor/polling.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//  +build !freebsd,!windows,!darwin
+//go:build !freebsd && !windows && !darwin
+// +build !freebsd,!windows,!darwin
 
 package monitor
 

wgengine/router/router_default.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !windows && !linux && !darwin && !openbsd && !freebsd
 // +build !windows,!linux,!darwin,!openbsd,!freebsd
 
 package router

wgengine/router/router_test.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build linux || windows
 // +build linux windows
 
 package router

wgengine/router/router_userspace_bsd.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build darwin || freebsd
 // +build darwin freebsd
 
 package router

wgengine/router/runner.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build linux
 // +build linux
 
 package router