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/pendopen.go

@@ -6,9 +6,7 @@ package wgengine
 
 import (
 	"fmt"
-	"os"
 	"runtime"
-	"strconv"
 	"time"
 
 	"tailscale.com/ipn/ipnstate"
@@ -22,17 +20,6 @@ import (
 
 const tcpTimeoutBeforeDebug = 5 * time.Second
 
-// debugConnectFailures reports whether the local node should track
-// outgoing TCP connections and log which ones fail and why.
-func debugConnectFailures() bool {
-	s := os.Getenv("TS_DEBUG_CONNECT_FAILURES")
-	if s == "" {
-		return true
-	}
-	v, _ := strconv.ParseBool(s)
-	return v
-}
-
 type pendingOpenFlow struct {
 	timer *time.Timer // until giving up on the flow