worker exiting -> worker stopping, to match RELAY_STOPPING
Make DIE_RC to control_exit() instead of exit().

control_exit() doesn't die if state is RELAY_RUNNING.
Using errno, so <errno.h>
Make failures call FATAL which either dies (if starting), or stops (if running).

Explicitly return failure after FATAL because now death is asynchronous.

Remove proctitle_destroy() because there's a race and nobody cares
about the leak if the process is shutting down.