[FEATURE]: add (proper) diff support to the settings module

[hrtmhood]

Describe the solution you'd like
Currently when using --diff with the settings modules, it is only displayed that a change would be made, and when in 'check' mode, the current setting are printed. When applying settings only the resultant settings are printed.

--diff should display the before AND after which either would be applied (when using --check) or actually was applied (when not using --check).

Describe alternatives you've considered
There is no reasonable alternative

[hrtmhood]

This appears to apply to many other modules too. --diff should show which parameter values have been, or would be, changed, in all cases.