envknob: add new package for all the strconv.ParseBool(os.Getenv(..))

A new package can also later record/report which knobs are checked and
set. It also makes the code cleaner & easier to grep for env knobs.

Change-Id: Id8a123ab7539f1fadbd27e0cbeac79c2e4f09751
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

scripts/check_license_headers.sh

@@ -10,7 +10,7 @@
 check_file() {
     got=$1
 
-    for year in `seq 2019 2021`; do
+    for year in `seq 2019 2022`; do
         want=$(cat <<EOF
 // Copyright (c) $year Tailscale Inc & AUTHORS All rights reserved.
 // Use of this source code is governed by a BSD-style