[Rd] exporting isChild in package parallel

[Michael Spiegel]

I was wondering if there is any interest in making the function
isChild() exported from the package parallel? This would be of great
help to anyone writing R packages that use thread-level parallelism,
and would like to throttle the spawning of threads whenever the R
process is detected to be a child process that was constructed by
mcfork().

Cheers,
--Michael