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

Updates #17852

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

net/captivedetection/captivedetection.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"tailscale.com/net/netmon"
+	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 )
@@ -32,7 +33,7 @@ type Detector struct {
 	// currIfIndex is the index of the interface that is currently being used by the httpClient.
 	currIfIndex int
 	// mu guards currIfIndex.
-	mu sync.Mutex
+	mu syncs.Mutex
 	// logf is the logger used for logging messages. If it is nil, log.Printf is used.
 	logf logger.Logf
 }