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

Updates #17852

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

net/dns/manager_windows.go

@@ -16,7 +16,6 @@ import (
 	"slices"
 	"sort"
 	"strings"
-	"sync"
 	"syscall"
 	"time"
 
@@ -27,6 +26,7 @@ import (
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/envknob"
 	"tailscale.com/health"
+	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/syspolicy/pkey"
@@ -51,7 +51,7 @@ type windowsManager struct {
 
 	unregisterPolicyChangeCb func() // called when the manager is closing
 
-	mu      sync.Mutex
+	mu      syncs.Mutex
 	closing bool
 }