Cleanup/refactor: Add new BLI_string_util.

Things like `BLI_uniquename` had nothing, but really nothing to do in
BLI_path_util files!

Also, got rid of length limitation in `BLI_uniquename_cb`, we can use
alloca here to avoid overhead of malloc while keeping free size (within
reasonable limits of course).

source/blender/makesrna/intern/rna_pose.c

@@ -40,6 +40,7 @@
 #include "DNA_scene_types.h"
 
 #include "BLI_math.h"
+#include "BLI_string_utils.h"
 
 #include "BLT_translation.h"