Do not set a priority for non-realtime threads

Do not set a priority for non-realtime threads

Contrary to process priority increment ("niceness"), thread priority
increment increase in the priority of the thread. So multiplying
negative priorities with -1 was totally wrong here.

On Linux, there is only one non-realtime (SCHED_OTHER) priority, 0, so
the code was failing safe silently. Don't know on other systems.