Replace 0 with NULL constant.
Moved joystick settings into the main settings page.

No major functionality changes.
Altered text to use 'controllers' instead of 'joysticks' to be more generic.
Merge pull request #845 from Susurrus/joystick2

Second pass at moving joystick settings into general settings dialog window

This has been sitting for comment for a couple days. Code review is good and change makes sense. Merging.