add env variable `OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX`
which is by default:
`OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX="@drive"`
so `/webdav/` -> points to user's personal drive (**backward compatibilit**y)
`/web/dav/@drive/{uuid|drive name}/` points to the respective drive
if admins want directly `/webdav/` pointing to list of drives they need to:
`OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX=""`
+ ensure lock is per user (RFC 4918 §9.11)
fix: #554
Edouard Vanbelle