ZSTD CLI: Use buffered output

[yoniko]

When opening an output file the zstd CLI will try to set it to buffered mode with a 1MB buffer.
Testing shown possible performance improvement on linux based machine and a major performance improvement on macOS:

comment                    dev  setvbuf  improvement
input_file      machine
enwik8.zst      desktop  0.210    0.211    -0.476190
                macbook  0.468    0.217    53.632479
                server   0.229    0.221     3.493450
silesia.tar.zst desktop  0.387    0.386     0.258398
                macbook  0.870    0.331    61.954023
                server   0.419    0.409     2.386635

Note: as per code comment, this usage of setvbuf might not be supported on legacy systems, but even in this case it shouldn't impact overall success of the execution.

[Cyan4973]

Looks good to me !