Lint code

[Chocobo1]

• Use proper printf specifier for unsigned integer type
• Enable more compiler warnings
• Avoid using gnu printf specifier
• Use proper initializer
• Add fallthrough comment to switch cases
• Use labs()

Note that there are still some compiler warnings left unhanded: https://github.com/jirka-h/haveged/pull/43/checks?check_run_id=820836997#step:5:11 I consider those are best left to the project owner to review/fix.

[jirka-h]

Thanks a lot for fixing these problems!

I will review the remaining ones. I have filled an issue to track the work progress:

https://github.com/jirka-h/haveged/issues/45