hhvm logging and abi improvements

* Backport /usr/lib/hphp/extensions/current symlink management from prod
* Make hhvm log destination configurable
** Set to /vagrant/logs for vagrant
** Set to /var/log for labs-vagrant
* Save core dumps to hhvm log directory
* Misc hhvm config settings to match prod defaults

Change-Id: Ie93a1a6c61bf8319b11f708e9fe86682b069edae