all: update tools that manage copyright headers

Update all code generation tools, and those that check for license
headers to use the new standard header.

Also update copyright statement in LICENSE file.

Fixes #6865

Signed-off-by: Will Norris <will@tailscale.com>

scripts/check_license_headers.sh

@@ -9,17 +9,14 @@
 check_file() {
     got=$1
 
-    for year in `seq 2019 2023`; do
-        want=$(cat <<EOF
-// Copyright (c) $year Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
+    want=$(cat <<EOF
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
 EOF
-        )
-        if [ "$got" = "$want" ]; then
-            return 0
-        fi
-    done
+    )
+    if [ "$got" = "$want" ]; then
+        return 0
+    fi
     return 1
 }
 
@@ -50,7 +47,7 @@ for file in $(find $1 -name '*.go' -not -path '*/.git/*'); do
             # Generated syscall wrappers
         ;;
         *)
-            header="$(head -3 $file)"
+            header="$(head -2 $file)"
             if ! check_file "$header"; then
                 fail=1
                 echo "${file#$1/} doesn't have the right copyright header:"