Some updates to README-OPENCL. This whole file should be reworked but
I am not the man.
Reduce global work size to 'count' in more OpenCL formats, speeding up self-test
and helping Single mode a lot. Also some bugfixes. And unify the various non-split
pbkdf2 kernels we had, so there is only one to maintain. RAR is currently b0rken.