Add "--limit-rate " option to curl. Download-only option not needing sudo in installinstallmacos.py

[porteusconf]

See munki#51 where I originally submitted this request to add 2 trivial features.

[grahampugh]

I'm just trying to understand the use case for these requests?

1. --download-only - what could you do with the downloaded files that wouldn't eventually need sudo, or that would be run from a management tool as root anyway? Who is the likely user who is downloading macOS installers but doesn't have admin rights?
2. --limit-rate 100k - I'd be suspicious of the success rate of a download that took several days. I'd be interested in hearing about tests before seeing a PR for this.