mdbx.NoSubdir: seems doesn't do anything

```
func TestNoSubdir(t *testing.T) {
tmpdir := t.TempDir()
db := mdbx.New(kv.TemporaryDB, log.Root()).InMem(tmpdir).Flags(func(u uint) uint {
return u | mdbx2.NoSubdir
}).MustOpen()
filepath.WalkDir(tmpdir, func(path string, d fs.DirEntry, err error) error {
fmt.Printf("NoSubdir: %s\n", path)
return nil
})
db.Close()

db = mdbx.New(kv.TemporaryDB, log.Root()).InMem(tmpdir).Flags(func(u uint) uint {
return u
}).MustOpen()
filepath.WalkDir(tmpdir, func(path string, d fs.DirEntry, err error) error {
fmt.Printf("Usual: %s\n", path)
return nil
})
db.Close()
}
```

```
=== RUN TestNoSubdir
NoSubdir: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001
NoSubdir: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-3296331714
NoSubdir: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-3296331714/mdbx.dat
NoSubdir: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-3296331714/mdbx.lck
Usual: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001
Usual: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-738347201
Usual: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-738347201/mdbx.dat
Usual: /var/folders/71/g_vc_lt50l983f5m2sy6ccxr0000gn/T/TestNoSubdir1607188179/001/erigon-memdb-738347201/mdbx.lck
--- PASS: TestNoSubdir (0.03s)
PASS
```