Changed nbThreads for nbWorkers

This makes it easier to explain that nbWorkers=0 --> single-threaded mode,
while nbWorkers=1 --> asynchronous mode (one mode thread on top of the "main" caller thread).
No need for an additional asynchronous mode flag.
nbWorkers>=2 works the same as nbThreads>=2 previously.

programs/bench.h

@@ -22,7 +22,7 @@ int BMK_benchFiles(const char** fileNamesTable, unsigned nbFiles, const char* di
 /* Set Parameters */
 void BMK_setNbSeconds(unsigned nbLoops);
 void BMK_setBlockSize(size_t blockSize);
-void BMK_setNbThreads(unsigned nbThreads);
+void BMK_setNbWorkers(unsigned nbWorkers);
 void BMK_setRealTime(unsigned priority);
 void BMK_setNotificationLevel(unsigned level);
 void BMK_setSeparateFiles(unsigned separate);