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>

net/dns/debian_resolvconf.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 || openbsd
 // +build linux freebsd openbsd
 
 package dns

net/dns/ini.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
 // +build windows
 
 package dns

net/dns/ini_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 windows
 // +build windows
 
 package dns

net/dns/manager_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 !linux && !freebsd && !openbsd && !windows
 // +build !linux,!freebsd,!openbsd,!windows
 
 package dns

net/dns/nm.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 dns

net/dns/openresolv.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 || openbsd
 // +build linux freebsd openbsd
 
 package dns

net/dns/resolvconf.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 || openbsd
 // +build linux freebsd openbsd
 
 package dns

net/dns/resolved.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 dns

net/dns/resolver/macios_ext.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 && ts_macext) || (ios && ts_macext)
 // +build darwin,ts_macext ios,ts_macext
 
 package resolver

net/dns/resolver/neterr_other.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 && !windows
 // +build !darwin,!windows
 
 package resolver