add -Wextra and remove a lot of warnings for C code

[uecker]

This is for people who need / want the C version of the code.

[stephanecharette]

What is the difference between this PR and the previous one PR #8915 ?

[uecker]

None really, the previous one was based on the master branch of my fork and it seems Github still does not allow one to change this for a pull request, so I closed the old one and recreated it using a new branch. In my fork I will go on and remove a lot of things to make a minimalistic C library with just the core functionality.

[stephanecharette]

I want to reiterate for anyone else who makes it here that the last few years worth of bug fixes, CUDA/cuDNN crash fixes, performance enhancements, etc..., are only in the new Darknet/YOLO repo here: https://github.com/hank-ai/darknet#table-of-contents

[cenit]

@stephanecharette I already told you to please avoid spamming everywhere your repo, the link in the readme is more than enough. Also because "last few years" is maybe an exaggeration, commits are still made here, but still this repo does not receive all the love it might deserve. In any case, I thank you @uecker for the contribution, and I'll look forward to merge it soon (after more review needed, of course)