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

Updates #17852

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

ipn/ipnlocal/local.go

@@ -247,7 +247,7 @@ type LocalBackend struct {
 	extHost *ExtensionHost
 
 	// The mutex protects the following elements.
-	mu sync.Mutex
+	mu syncs.Mutex
 
 	// currentNodeAtomic is the current node context. It is always non-nil.
 	// It must be re-created when [LocalBackend] switches to a different profile/node
@@ -329,14 +329,14 @@ type LocalBackend struct {
 	//
 	// tkaSyncLock MUST be taken before mu (or inversely, mu must not be held
 	// at the moment that tkaSyncLock is taken).
-	tkaSyncLock sync.Mutex
+	tkaSyncLock syncs.Mutex
 	clock       tstime.Clock
 
 	// Last ClientVersion received in MapResponse, guarded by mu.
 	lastClientVersion *tailcfg.ClientVersion
 
 	// lastNotifiedDriveSharesMu guards lastNotifiedDriveShares
-	lastNotifiedDriveSharesMu sync.Mutex
+	lastNotifiedDriveSharesMu syncs.Mutex
 
 	// lastNotifiedDriveShares keeps track of the last set of shares that we
 	// notified about.