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

Updates #17852

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

net/dns/resolver/forwarder.go

@@ -37,6 +37,7 @@ import (
 	"tailscale.com/net/netx"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/net/tsdial"
+	"tailscale.com/syncs"
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/nettype"
@@ -231,7 +232,7 @@ type forwarder struct {
 	ctx       context.Context    // good until Close
 	ctxCancel context.CancelFunc // closes ctx
 
-	mu sync.Mutex // guards following
+	mu syncs.Mutex // guards following
 
 	dohClient map[string]*http.Client // urlBase -> client