util: add parse fallback helpers (#20022)

util/def: add def.Bool and def.Duration default parse helpers

Replace multiple instances of def.Bool and def.Duration with a new util/def
package.

Updates #20018

Co-authored-by: Bobby <boby@codelabs.co.id>
Co-authored-by: Simon Law <sfllaw@tailscale.com>
Signed-off-by: Bobby <boby@codelabs.co.id>
Signed-off-by: Simon Law <sfllaw@tailscale.com>

feature/debugportmapper/debugportmapper.go

@@ -11,7 +11,6 @@ import (
 	"net"
 	"net/http"
 	"net/netip"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -20,6 +19,7 @@ import (
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/portmapper"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/def"
 	"tailscale.com/util/eventbus"
 )
 
@@ -66,7 +66,7 @@ func serveDebugPortmap(h *localapi.Handler, w http.ResponseWriter, r *http.Reque
 		}
 	}
 
-	if defBool(r.FormValue("log_http"), false) {
+	if def.Bool(r.FormValue("log_http"), false) {
 		debugKnobs.LogHTTP = true
 	}
 
@@ -191,14 +191,3 @@ func serveDebugPortmap(h *localapi.Handler, w http.ResponseWriter, r *http.Reque
 		}
 	}
 }
-
-func defBool(a string, def bool) bool {
-	if a == "" {
-		return def
-	}
-	v, err := strconv.ParseBool(a)
-	if err != nil {
-		return def
-	}
-	return v
-}