HHVM: use syslog; route to fluorine

* Configure HHVM to use syslog for logging.
* Add HHVM to MediaWiki's rsyslog.conf.

This leaves the file path and description of the rsyslog config file somewhat
misnamed (since it's no longer Apache specific), but I'll clean that up in
another patch.

Change-Id: I4590fdcf1738d7c839098580858d4bd42098fc47