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

Updates #17852

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

net/netutil/netutil.go

@@ -8,7 +8,8 @@ import (
 	"bufio"
 	"io"
 	"net"
-	"sync"
+
+	"tailscale.com/syncs"
 )
 
 // NewOneConnListener returns a net.Listener that returns c on its
@@ -29,7 +30,7 @@ func NewOneConnListener(c net.Conn, addr net.Addr) net.Listener {
 type oneConnListener struct {
 	addr net.Addr
 
-	mu   sync.Mutex
+	mu   syncs.Mutex
 	conn net.Conn
 }