This fixes some minor issues that I noticed and fixed when adding
partitioning to monitor.
* Trap both INT as well as TERM. This allows you to use Ctrl+C to
cleanly shutdown a respirate process started manually.
* On apoptosis, exit 2 instead of 0, since I don't think apoptosis
indicates success.
* Avoid a potential race condition, where Kernel#exit is called
concurrently by multiple threads. I don't think this matters on
CRuby, but it's better to only exit in a single place, and have
the thread update a variable to indicate clean shutdown.
1244412bc9