[RFC] Replace TRUE/FALSE with stdbool (Part 1).

This is an ongoing work to replace TRUE/FALSE with stdbool on a per-file basis.

I will also change the comments to follow the style guide.

This PR can be merged anytime, to prevent future merge conflicts. Each commit will contain the changes for the complete file and all the files, affected by the changes.
Can use `@return` after the `@param` list.