Clean up AXParameter callback registration code

[killenheladagen]

• The ax_parameters[] array has been given a name reflecting its purpose.
• The array is now NULL-terminated, to simplify looping over it.
• The AXParameter callback function has been given a name reflecting its purpose.
• The unnecessary qualification of the parameter name when registering the callback has been removed.

Checklist before requesting a review

• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [ ] I have made corresponding changes to the documentation