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/fileio.h

@@ -54,7 +54,7 @@ void FIO_setDictIDFlag(unsigned dictIDFlag);
 void FIO_setChecksumFlag(unsigned checksumFlag);
 void FIO_setRemoveSrcFile(unsigned flag);
 void FIO_setMemLimit(unsigned memLimit);
-void FIO_setNbThreads(unsigned nbThreads);
+void FIO_setNbWorkers(unsigned nbWorkers);
 void FIO_setBlockSize(unsigned blockSize);
 void FIO_setOverlapLog(unsigned overlapLog);
 void FIO_setLdmFlag(unsigned ldmFlag);