Feature suggestion: allow passing "--rsyncable" to zstd compressor

[goverp]

With luck, rsyncing a file generated with gensquashfs may save on transferring the entire file. For example, I use rsync to maintain my snapshot of the Gentoo portage tree (order of 100 MB compressed using lzop), and weekly changes used to need a transfer of about 60% of the file (for some reason, that's no longer true, but that's a Gentoo issue, and anyway it's irrelevant).

It struck me that as zstd has an explicit option "--rsyncable" to make its output more rsync-friendly, it would benefit gensquashfs to be able to specify it, but "gensquashfs -c sztd -X help" just says "level=" as the list of available options (at least, on my Gentoo system).