Bug#1080322: latexmk: add -parallel=N option

I asked github copilot to add a latexmk -parallel[=n] option and ate
dinner and came back and tested it and it seems to work fine. First it
just allowed various top-level .tex files to be processed in parallel.
Then I asked for other stuff to be  concurrent too, and also to stop
mixing up outputs and buffer them instead, and it did that. Then I
told it I was worried about errors from interaction of multiple
subjobs generating the same intermediate file, so it added some
flock'ing. And I tested it some more and it seems to work fine.

https://github.com/barak/latexmk branch copilot/add-parallel-option-to-latexmk

"If you can dream it...!"

--Barak.