add (hidden) option to always show smb root as writable

some smb servers are very insistent in reporting that the root of the share is readonly, even if it isn't.

This works around the problem by adding a hidden option to overwrite the permissions of the root of the share.

This can be enabled using

```bash
occ files_external:config <mount id> root_force_writable true
```

where you can find your mount id using

```bash
occ files_external:list
```

Signed-off-by: Robin Appelman <robin@icewind.nl>