Several fixes for pgbench-tools

The main fixes here are:
 * Make sure to properly kill subprocesses launched through timed-os-stats. I noticed on a Debian lenny machine that dozens of vmstat and iostat child processes were hanging around after ./runset, with only the parent timed-os-stats process actually killed off. I believe I have a portable fix for this, albeit with the cost of yet another Python script.
 * Use foreign keys in resultdb.sql