macOS build fix: no SOCK_CLOEXEC on darwin.

We can't use the sysdeps.h copy of this code, so for now add yet
another copy & paste. We should come up with something better in
libbase, but let's get the macOS build fixed first.

Test: builds on mac
Change-Id: Ifb059990d7091f3d91b54e684f4ee14afe69c28b