tty: stdio properties defined inside exec() child

Properties inside exec() child were not predefined because process.stdin is a PIPE.

This commit suggests predefining them inside getStdin() function.

Fixes: https://github.com/nodejs/node/issues/2333

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)