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

Updates #17852

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

util/syspolicy/rsop/resultant_policy.go

@@ -7,10 +7,10 @@ import (
 	"errors"
 	"fmt"
 	"slices"
-	"sync"
 	"sync/atomic"
 	"time"
 
+	"tailscale.com/syncs"
 	"tailscale.com/util/syspolicy/internal/loggerx"
 	"tailscale.com/util/syspolicy/setting"
 	"tailscale.com/util/testenv"
@@ -58,7 +58,7 @@ type Policy struct {
 
 	changeCallbacks policyChangeCallbacks
 
-	mu             sync.Mutex
+	mu             syncs.Mutex
 	watcherStarted bool // whether [Policy.watchReload] was started
 	sources        source.ReadableSources
 	closing        bool // whether [Policy.Close] was called (even if we're still closing)