Skippable Frames in zstd files

[Jamboretz, Chris]

Hi Antonio,

The zstd compression format allows for skippable frames
https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#skippable-frames

pzstd uses this and puts the skippable frame at the beginning of the file. 

The function test_format in zutils.cc doesn't take this magic_number into 
account and so reports the file as uncompressed.

Thank you,
Chris Jamboretz