Duke3D: Add a confirmation dialog that notifies the user that a keyboard key is already bound

This is currently restricted to the keyboard binds menu. How it looks ingame:

The dialog provides three options:

• Y -> Assign key and clear existing keybinds for that key
• N -> Assign key, do not clear existing keybinds for that key
• Escape -> Cancel, do not assign key

This addresses issue #164 (closed) and supersedes MR !150 (closed).

Additional changes:

• Add a cvar cl_kbconfirm that, if set to 0, will remove the confirmation dialog and make the menu behave as before. (for people that prefer not seeing this dialog pop up)
• Prevent assigning the same key to both columns of a single function. If done, now the second column will be erased.