zstd cli can now compress using multi-threading

added : command -T#
added : ZSTD_resetCStream() (zstdmt_compress)
added : FIO_setNbThreads()  (fileio)

programs/fileio.h

@@ -40,6 +40,7 @@ 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);
 
 
 /*-*************************************