This change drops in KMP_DEVICE_THREAD_LIMIT to replace KMP_MAX_THREADS. It's possible there will eventually be a OMP_DEVICE_THREAD_LIMIT, and we need something to distinguish from OMP_THREAD_LIMIT, which is currently implemented wrong (the fix for that will be added soon in a separate patch). KMP_ALL_THREADS is deprecated here, but we can keep the "all" option on KMP_DEVICE_THREAD_LIMIT to support that functionality. KMP_DEVICE_THREAD_LIMIT now has priority over its deprecated rival KMP_ALL_THREADS. I also cleaned up some comments that incorrectly referred to non-existent kmp_max_threads variable instead of kmp_max_nth.
I've left the name of where this setting eventually ends up as __kmp_max_nth, for now.
This change does not change much in the way of functionality. It does NOT change OMP_THREAD_LIMIT. It's just cleaning up and setting up for that.