add reshare option for the OCS Share API

Add a reshare option to the OCS Share API. Usage:

```
/ocs/v1.php/apps/files_sharing/api/v1/shares?path=file.txt&reshares
```

Will not only return the shares from the current user for ```file.txt``` but also possible reshares.

cc @karlitschek @PVince81 @DeepDiver1975