Add support for Mac OS X.

Incredibly, this was the only piece missing. This does the default
IP detection using the Mac OS X tools. We check for the Darwin platform,
otherwise assume Linux to minimize any weird platform strings breaking
something. Part of RR-112.

Change-Id: Id84a300e15cb61abaedcde0c9d2f87fc138f9352
Merge "Add support for Mac OS X."