serve nfs: make metadata files have special file handles

Metadata files have the file handle of their source file with
0x00000001 suffixed in big endian so we can look them up directly from
their file handles.

cmd/nfsmount/nfsmount_test.go

@@ -11,6 +11,8 @@ import (
 	"testing"
 
 	"github.com/rclone/rclone/cmd/serve/nfs"
+	"github.com/rclone/rclone/fs/object"
+	"github.com/rclone/rclone/vfs"
 	"github.com/rclone/rclone/vfs/vfscommon"
 	"github.com/rclone/rclone/vfs/vfstest"
 	"github.com/stretchr/testify/require"
@@ -38,7 +40,7 @@ func TestMount(t *testing.T) {
 			nfs.Opt.HandleCacheDir = t.TempDir()
 			require.NoError(t, nfs.Opt.HandleCache.Set(cacheType))
 			// Check we can create a handler
-			_, err := nfs.NewHandler(context.Background(), nil, &nfs.Opt)
+			_, err := nfs.NewHandler(context.Background(), vfs.New(object.MemoryFs, nil), &nfs.Opt)
 			if errors.Is(err, nfs.ErrorSymlinkCacheNotSupported) || errors.Is(err, nfs.ErrorSymlinkCacheNoPermission) {
 				t.Skip(err.Error() + ": run with: go test -c && sudo setcap cap_dac_read_search+ep ./nfsmount.test && ./nfsmount.test -test.v")
 			}