First version of --devices=N[,..] parsing. It fills an array ocl_device_list[MAXGPUS] but currently nothing is doing anything with that array.

The code blocks in john.c that do this should ideally move out to cuda_init() and
start_opencl_devices() (or some other function).

Signed-off-by: Claudio André <claudio.andre@correios.net.br>