Remove tmp_* files after upload

During upload tmp_* files are created which remain in the temporary directory after upload. imce_copy_uploaded() copies the temp file (imce.module line 302), which leaves the tmp_ file in the temporary directory.
One solution is to replace copy() by rename(). The other solution is to provide a hook_cron() implementation to clean-up the tmp_ files. The attached patch does this for 5.x-1.x-dev.