Cologne SG1: is std::process thread safe

[JeffGarland]

We should note it as an open question, whether std::process should be thread safe.

The question started with the a discussion of if multiple threads could call 'wait' and the fact that it might update the object means synchronization. Also suggested that wait could be implemented as wait_for(0) to improve this (Chris K suggested).

After more discussion of posix and other apis seemed that the general conclusion is that it doesn't need to be thread safe.

Not adding to anything to the paper just yet.