feat(drive): fix webdav back-compat

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

tests/api/run.sh

@@ -185,6 +185,8 @@ hurl --variables-file "$API_DIR/test.env" --file-root "$REPO_ROOT/tests" --test
   "$API_DIR/cross_drive_move.hurl" \
   "$API_DIR/cross_drive_copy.hurl" \
   "$API_DIR/webdav_dead_properties.hurl" \
+  "$API_DIR/webdav_drive_root.hurl" \
+  "$API_DIR/webdav_permissions.hurl" \
   "$API_DIR/webdav_nested_move_cascade.hurl" \
   "$API_DIR/wopi_authz.hurl"