zstdmt cli and API allow selection of section sizes

By default, section sizes are 4x window size.
This new setting allow manual selection of section sizes.
The larger they are, the (slightly) better the compression ratio,
but also the higher the memory allocation cost,
and eventually the lesser the nb of possible threads,
since each section is compressed by a single thread.

It also introduces a prototype to set generic parameters,
ZSTDMT_setMTCtxParameter()

The idea is that it's possible to add enums
to extend the list of parameters that can be set this way.
This is more long-term oriented than a fixed-size struct.
Consider it as a test.

programs/fileio.h

@@ -41,6 +41,7 @@ void FIO_setChecksumFlag(unsigned checksumFlag);
 void FIO_setRemoveSrcFile(unsigned flag);
 void FIO_setMemLimit(unsigned memLimit);
 void FIO_setNbThreads(unsigned nbThreads);
+void FIO_setBlockSize(unsigned blockSize);
 
 
 /*-*************************************