Parallel xzcat written for virt-builder.

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PLEASE NOTE that I wrote this code to experiment with parallel lzma.
The code here was eventually included in virt-builder, see:

https://github.com/libguestfs/libguestfs/blob/master/builder/pxzcat-c.c

If you are interested in a general purpoae parallel xz, then I
recommend using the following project instead, which can do both
decompression and compression:

https://github.com/vasi/pixz