Tweaks to HHVM module

* Set hhvm.log.header = true, as suggested by Filippo
* Tell upstart to use SIGHUP as the kill signal for HHVM, which will make it
  perform a graceful stop.
* Correct path to hstr and FastCGI config dir.

Change-Id: I06a7a825b9f1c6538e7da3c62ef4869b32f2058c
Merge "Tweaks to HHVM module" into production