Fix stream management to work with new packages

Recent packages (oracle mysql 5.6) have some strange behaviour related
to stdout/stderr. Switching the streams to stdout only for the apt-calls
appears to resolve this.

Failed package installs should still be caught by the exit code checking
at the end of the install.
This can just return size_t as it's result.

As for how to know when the process is finished, don't worry- the "wait_forever_for_exit()" you're calling causes "is_finished" to be true, which lets client code know EOF was reached.