Re-allow communication with SubProcess through stdIO.

We have tests that rely on being able to communicate with sub-
-processes through stdin and stdout. Simply letting the subprocess
inheriting the IO streams from the parent process does not allow
such communication.
This changeset reverts the change that made SubProcess inherit IO.
Merge pull request #2828 from thobe/2.2-revert-forwarding-SubProcess-streams

Re-allow communication with SubProcess through stdIO