Blender Git Loki

Cleanup: use BLI_path_join in appdir.c to simplify logic

Change `test_path` function so:

- Joining paths calls a function instead of being performed inline.
- Optional NULL paths must always be ordered last (easier to follow).

Other minor changes:

- Remove FIXME comment is it's handled properly by BLI_path_join.
- The wrong size was being used copying into `targetpath`.