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

Updates #17852

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

proxymap/proxymap.go

@@ -9,9 +9,9 @@ import (
 	"fmt"
 	"net/netip"
 	"strings"
-	"sync"
 	"time"
 
+	"tailscale.com/syncs"
 	"tailscale.com/util/mak"
 )
 
@@ -22,7 +22,7 @@ import (
 // ask tailscaled (via the LocalAPI WhoIs method) the Tailscale identity that a
 // given localhost:port corresponds to.
 type Mapper struct {
-	mu sync.Mutex
+	mu syncs.Mutex
 
 	// m holds the mapping from localhost IP:ports to Tailscale IPs. It is
 	// keyed first by the protocol ("tcp" or "udp"), then by the IP:port.