Use `R_isForkedChild` to informatively error in multicore scenarios

[DavisVaughan]

Ark does not support multicore parallel processing, but we don't have a good way to tell the user this

I recently stumbled across

https://github.com/wch/r-source/blob/69a41188dbe32760d147bb9da6e263e5dbd21071/src/include/Defn.h#L1655

which I think we have access to.

We could check that at every top level repl iteration, and try and log or informatively error if it is ever TRUE, because ark is very much not supported in multicore mode right now (and probably never will be).

[lionel-]

We could check that at every top level repl iteration

IIUC only the children will have this set, and these do not run a REPL, so our frontend REPL handlers will never be called.

On the other hand polled events are run: https://github.com/r-devel/r-svn/blob/671453f7c085d503dff55b533797f58d789e5510/src/unix/sys-unix.c#L1173-L1178

So we could check from our polled events handler. We could even force an exit() from there after logging. However I'm not sure how we could reliably communicate with the parent process to have it exit as well, which would be ideal since all our internal state is probably borked (e.g. 0mq is not fork-safe).

Maybe we should just hook into the parallel package and disable forking there, or use a fallback behaviour (socket/sequential).