Fix keybinding not being available to override through configuration settings.

Fix keybinding not being available to override through configuration settings.

In the function changemapping() the linked list of keybindings is traversed in
a way the last element of the list (tail) wont be processed to rebind.
The loop iterates over all elements of the linked list until an element
which has none _next_ to it, ie: the last one, efectibly skipping the last element
in the list.

The changes fix this by iterating over all elements until finding a NULL element.

Patch by desyncr via GitHub:
https://github.com/desyncr/vimprobable/commit/050a96f849e7b14ba2e2629268ee559c5afe6856