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>

wgengine/magicsock/magicsock.go

@@ -61,8 +61,8 @@ import (
 // useDerpRoute reports whether magicsock should enable the DERP
 // return path optimization (Issue 150).
 func useDerpRoute() bool {
-	if debugUseDerpRouteEnv != "" {
-		return debugUseDerpRoute
+	if b, ok := debugUseDerpRoute.Get(); ok {
+		return b
 	}
 	ob := controlclient.DERPRouteFlag()
 	if v, ok := ob.Get(); ok {