bug#45198: 28.0.50; Sandbox mode

[Stefan Monnier]

> namespace also don't work in multithreaded processes. Allowing
> sandboxing (at least for now) only at process startup avoids such
> issues and should be good enough for the given use case (Flymake
> background compilation), since we need to start a new process anyway.

BTW, while flymake does already use a background process, I think
there's a deeper reason why we'll always need a separate process:
there's no way to exit a sandbox.


        Stefan