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

Updates #17852

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

wgengine/magicsock/blockforever_conn.go

@@ -10,11 +10,13 @@ import (
 	"sync"
 	"syscall"
 	"time"
+
+	"tailscale.com/syncs"
 )
 
 // blockForeverConn is a net.PacketConn whose reads block until it is closed.
 type blockForeverConn struct {
-	mu     sync.Mutex
+	mu     syncs.Mutex
 	cond   *sync.Cond
 	closed bool
 }