opencl: remove cl_kernels structure definition and mention.

No longer used.

TODO: check if clState would benefit from having kernel name.
misc: add comment on what cgpu_info->{,k}name are.
opencl: rename queue_scrypt_kernel() to queue_kernel().

There are no other kernels, and differentiating kernels in the future
should be done in a universal way, preferably not by choosing a function.

Note how queue_kernel() is used (passed as function reference).
opencl: rename cgpu_info->kname to cgpu_info->kernelname.
build: remove kernel name definitions from configure.ac.
opencl: add log notice, change another to notice level, shorten comment.
misc: suggest checking configuration in log message.
core: check if GPU thread is disabled before checking if it's waiting on getwork.

Otherwise the watchdog thread segfaults on checking an uninitialised
thr->getwork value.