Add documentation for compressors and mksquashfs

[afbjorklund]

Document --mksquashfs-args and the different compressor options...

Hopefully compatibility is less of an issue with bundled mksquashfs.

Description of the Pull Request (PR):

Documents common options to the new --mksquashfs-args backdoor.

For instance how to change processors or compressors, from -help.

This fixes or addresses the following GitHub issues:

• Fixes Document --mksquashfs-args #308

[afbjorklund]

Note that both lz4 and zstd programs offer a "benchmark" feature.

This can be used, in combination with -no-compression compressor...

$ apptainer build --mksquashfs-args="-no-compression" python.sif docker://python

$ lz4 -b1 python.sif 
 1#python.sif        : 981495808 -> 495176737 (1.982), 628.3 MB/s ,3159.1 MB/s 
$ lz4 -b9 python.sif 
 9#python.sif        : 981495808 -> 405101750 (2.423),  34.4 MB/s ,3256.6 MB/s 

$ zstd -b1 python.sif
 1#python.sif        : 981495808 -> 377207154 (2.602), 399.3 MB/s ,1143.7 MB/s 
$ zstd -b9 python.sif
 9#python.sif        : 981495808 -> 298002825 (3.294),  32.9 MB/s ,1154.6 MB/s 

YMMV. (but note that compression levels only affects compression, not decompression)

Note: The SIF file needs to be uncompressed for the benchmark and comparison only.

Level 1 is "fast" and level 9 is "best". (level 15 is superslow, 7.13 MB/s)

mksquashfs calls lz4 level 9 with -Xhc. (HC as in "high compression")

[afbjorklund]

http://zstd.net/ versus gzip:

https://lz4.org/ versus gzip: