HHVM: keep DSO path up-to-date; rename script

* Rename hhvm-debug-dump => hhvm-dump-debug (makes the purpose clearer.)
* Avoid hard-coding path to HHVM binary; gdb will find it.
* Make upstart job refresh DSO symlink.

Change-Id: I109f92543b5e4ff8d8a265e90092098ff6d99341