crush: add CrushWrapper::rename_item and can_rename_item

crush: add CrushWrapper::rename_item and can_rename_item

The can_rename_item is a const method checking if renaming an item could
succeed. If not it returns a unique -errno code and a human readable
message message.

Trying to rename a non existent item into an existent item returns
-EALREADY which can be treated as success if renaming is to be
idempotent.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>