Make the compile work on a Mac

[ballanty]

This change:

• sed as used required the addition of a small change to have it work on the Mac. The Makefile was altered to detect the platform and conditionally use sed in different ways in one case.
• it looks like "packets" was once a buffer size measure and it was changed to bytes at some point. Fix what appears to be a leftover reference to packets.
• parsing of quantities ("2days" for example) seemed to differ by platform (Mac (didn't work) vs Linux (did)). Replaced parsing approach with one using regular expressions to make it more robust to platform differences between the way std::..stream operator>> parsed.