LinkedList.c: move isUserPointerValid() higher up in the file (to be usable by private functions for double pointers)
LinkedList.c: use isUserPointerValid(), exceptionally, in private function
LinkedList.c: use isUserPointerValid(), change variable names from p_node to cursor where appropriate
LinkedList.c: add private swapNodes() function
LinkedList.c: implement simpleSort as llist_bubbleSort()
LinkedList.c: move llistCursor_getData within the "cursor functions" boundary comments. Implement _setData() counterpart
LinkedList.h: fix prototypes as needed
LinkedList.c: partly revert 42b2550dd28cf120b5708ce240a633bf67f9623c

Turns out that setter functions to cursors shouldn't check whether
the cursor is initialized... (myderp)