slnkcctr.cpp: Portabilized `main` declaration

Now uses `main` declaration as in C++ standard and OpenCV tutorials,
e.g.

http://docs.opencv.org/doc/tutorials/highgui/video-input-psnr-ssim/video-input-psnr-ssim.html
slnkcctr.cpp: Reformatted
Displays video stream from camera

Tutorial used:
http://docs.opencv.org/doc/tutorials/highgui/video-input-psnr-ssim/video-input-psnr-ssim.html