BLI_path_suffix() - new path util functon to add a suffix to a filepath (before the extension)

Patch properly tested and working. Campbell should I use BLI_strcpy and BLI_sprintf here? They don't seem necessary here, but I don't know.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D465
changes to b2ffdf99 (BLI_path_suffix()) - to be squashed