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>

cmd/tailscale/cli/file.go

@@ -16,7 +16,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"strconv"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -26,6 +25,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/client/tailscale/apitype"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
@@ -148,7 +148,7 @@ func runCp(ctx context.Context, args []string) error {
 				name = filepath.Base(fileArg)
 			}
 
-			if slow, _ := strconv.ParseBool(os.Getenv("TS_DEBUG_SLOW_PUSH")); slow {
+			if envknob.Bool("TS_DEBUG_SLOW_PUSH") {
 				fileContents = &slowReader{r: fileContents}
 			}
 		}