[WIP] Using bool to represent boolean values.

As stated in style guide we must use bool from stdbool.h instead of TRUE/FALSE int constants.

What has been done:
- Used bool in os module.
- Added clint check for warning on TRUE/FALSE.

Usage of these constants in codebase is too heavy. So I will probably refactor just a few other (preferable new) code in this PR, so it will not get bloated.