Implement "-debug"

Any environment variable with the prefix "NVIDIA_REQUIRE_" will have
its value passed to nvidia-container-cli.

NVIDIA_CUDA_VERSION is no longer used and the CUDA images will be
replaced by a NVIDIA_REQUIRE_CUDA variable.

Those checks can be disabled globally in the configuration file or for
each container using NVIDIA_DISABLE_REQUIRE.

It gives the opportunity to perform cleanup in the main function.

Signed-off-by: NFelix Abecassis <fabecassis@nvidia.com>