quic/qlog: create log files with O_EXCL

Avoid confusing log corruption if two loggers try to write
to the same file simultaneously.

Change-Id: I3bfbcf56aa55c778ada0178d7c662c414878c9d1
Reviewed-on: https://go-review.googlesource.com/c/net/+/545577
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/quic/qlog/qlog.go

@@ -180,7 +180,7 @@ func newTraceWriter(opts HandlerOptions, info TraceInfo) (io.WriteCloser, error)
 		if !filepath.IsLocal(filename) {
 			return nil, errors.New("invalid trace filename")
 		}
-		w, err = os.Create(filepath.Join(opts.Dir, filename))
+		w, err = os.OpenFile(filepath.Join(opts.Dir, filename), os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0666)
 	} else {
 		err = errors.New("no log destination")
 	}