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

Updates #17852

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

control/controlbase/conn.go

@@ -18,6 +18,7 @@ import (
 
 	"golang.org/x/crypto/blake2s"
 	chp "golang.org/x/crypto/chacha20poly1305"
+	"tailscale.com/syncs"
 	"tailscale.com/types/key"
 )
 
@@ -48,7 +49,7 @@ type Conn struct {
 
 // rxState is all the Conn state that Read uses.
 type rxState struct {
-	sync.Mutex
+	syncs.Mutex
 	cipher    cipher.AEAD
 	nonce     nonce
 	buf       *maxMsgBuffer   // or nil when reads exhausted

control/controlclient/direct.go

@@ -23,7 +23,6 @@ import (
 	"runtime"
 	"slices"
 	"strings"
-	"sync"
 	"sync/atomic"
 	"time"
 
@@ -44,6 +43,7 @@ import (
 	"tailscale.com/net/netx"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tsdial"
+	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/tstime"
@@ -92,7 +92,7 @@ type Direct struct {
 
 	dialPlan ControlDialPlanner // can be nil
 
-	mu              sync.Mutex        // mutex guards the following fields
+	mu              syncs.Mutex       // mutex guards the following fields
 	serverLegacyKey key.MachinePublic // original ("legacy") nacl crypto_box-based public key; only used for signRegisterRequest on Windows now
 	serverNoiseKey  key.MachinePublic