Fixed STDOUT/STDERR IO streams do not exist in process isolation.

The PHP CLI SAPI does not auto-register the standard IO streams when a script is piped into STDIN (which is the case in process isolation); cf.

- https://bugs.php.net/bug.php?id=43283
- http://php.net/manual/en/features.commandline.io-streams.php
- https://gist.github.com/sun/d02c242514c8f34bccdb
Fixed unnecessarily restarted output buffer at end of process isolation script.
HHVM defines standard IO streams on CLI even if a script is injected via STDIN.
Merge pull request #1366 from sun/cli-io-streams

STDOUT/STDERR IO streams do not exist in process isolation
Merge pull request #1367 from sun/isolation-extra-ob

[minor] Process isolation script unnecessarily restarts output buffer after completion