[pve-devel] backup : add pigz compressor

[Eric Blevins]

> But pigz needs all you CPU power do be that fast! If you have running VMs,
> it will be as slow as normal gzip, or it will massively slow down the VMs.
> I am really afraid this will trigger many support calls ...

Make the default number of pigz CPUs 2, this should not have much of a
CPU impact even on servers with small number of cores.
Those of us who have dozens of cores can edit vzdump.conf and set an
option to use more cores for pigz.

You could even make it so using pigz requires a setting in
vzdump.conf. So in GUI you still can only select gzip or lzop.
If set to gzip and vzdump.conf has pigz:on then pigz is used instead of gzip.
Most novice users are only going to use the GUI, this would reduce the
likelyhood of them turning on pigz and then complaiing about their
decision.