OpenCL: Common support for multiple devices

RFC @claudioandre @Sayantan2048

We should figure out a KISS way of supporting multi-device use for more formats, with as little effort as possible (I'm lazy).

Brainstorming:

1. All common OpenCL variables/functions should work on arrays. Some already do. For example, local_work_size, global_work_size, crypt_kernel and so on should all be arrays of [MAX_GPU_DEVICES].
2. A format calling opencl_init would init all devices specified (I think this already happens)
3. Auto-tune should be made for all chosen devices, storing LWS/GWS results per device. Note: If we deem two or more devices identical, we should obviously just re-use the values and not test again.
4. Auto-tune can (already) aim for a duration time, eg. 200ms.
5. Say we end up with a global_work_size[0] of 1024 and global_work_size[1] of 2048. We'd simply set max_keys_per_crypt to 3072.
6. set_salt would send same salt to each device.
7. crypt_all() would iterate through the device list and send the number of keys to each device that corresponds with its global_work_size. Then launch each kernel, and so on.

That's about it, I think.

Start by tracing the OpenCL entry points named in the issue: opencl_init, set_salt, and crypt_all, along with shared variables such as local_work_size and global_work_size. Determine how device lists and auto-tuning currently work; done would mean a settled, tested design for distributing work and tuning across multiple devices.