syncs: add Mutex/RWMutex alias/wrappers for future mutex debugging

Updates #17852

Change-Id: I477340fb8e40686870e981ade11cd61597c34a20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -23,7 +23,6 @@ import (
 	"runtime"
 	"slices"
 	"strings"
-	"sync"
 	"sync/atomic"
 	"time"
 
@@ -44,6 +43,7 @@ import (
 	"tailscale.com/net/netx"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tsdial"
+	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/tstime"
@@ -92,7 +92,7 @@ type Direct struct {
 
 	dialPlan ControlDialPlanner // can be nil
 
-	mu              sync.Mutex        // mutex guards the following fields
+	mu              syncs.Mutex       // mutex guards the following fields
 	serverLegacyKey key.MachinePublic // original ("legacy") nacl crypto_box-based public key; only used for signRegisterRequest on Windows now
 	serverNoiseKey  key.MachinePublic