Fixes #10705. New function 'HelperFile::joinPathFilename($pre, $post)'. Joins only if $post is without leading slash.

splitPdf: using 'fileSplit=jpeg' and uploading a PDF with only one page, results in split.jpg. Now, split.jpg is renamed internally to split-0.jpg to provide a repair shell script a unified naming scheme.

Fixes #7925. a) change CWD during split reduced to splitting only. b) fixed problem in mkDirParent() with absolute paths, c) make logger independent of CWD, d) fixed problem with `mktemp --tmpdir`(difference Ubuntu 16 / 18) by using PHP function, e) fixed some 'undefined index' problems

Logger.php: remove UserAgent - that one is logged in FormSubmitLog Table. Add 'cookie' to filter individual actions.

F4901: implement split with 'convert' (jpeg). Refactor chmod(), unlink(), rename(), rmdir(), copy() to use by QFQ version which throws an exception.

Upload: rename 'chmod' to 'chmodFile'. Implement 'chmodDir'. Permissions applied for all new created directories.