parallel zstd

[curtis]

Using pzstd speeds up compression considerably on multi-core systems. And incurs only a minor size penalty.

I threw together this patch to try it out. ​https://github.com/dotysan/mc/commit/589227e13abb99fbe23667203eace3c37b4d12a9

It does pass make check but I haven't hand-tested all scenarios yet.

Anyone have strong thoughts?

Maybe it should be a separate menu item, in case there are really old systems without it? But it's been around since at least 2017.

Related to: #4571