Improvements to tolower

When running `tolower` I noticed two flaws this pull request fixes:
1. Previously `tolower` renamed all files, whether they already had been lowercase or not.
2. During renaming `tolower` renamed each file to a temporary file (named "`TMP_THIS_IS_A_VERY_TMP_NAME`") before renaming it from the temporary file to the original filename in lowercase. That could cause losing single files, when aborting `tolower`. Although I believe this behavior has been there with purpose I couldn't get why, so I removed it.

Both changes should increase the performance of the renaming while the second one should also avoid data loss.