xargs -P0 suport

[Nikolai Lifanov]

Hi current@!

Can someone take a look at Bugzilla #199976 please?
It's a pretty trivial patch that adds -P0 support to xargs.

If there is any feedback or the patch is wrong, I can rework it.
I've been running CURRENT with it for a bit now with no problems.

I would like very much for it to make it to 10.2-RELEASE.
It would make it much easier for me to convert some systems to FreeBSD
as many custom scripts that use it will be able to port as-is.

Thank you!

- Nikolai Lifanov