Typically, 0 denotes successful exit. In particular, init systems
will complain if another different value is returned. This presents a
problem for automated installation jobs which want to restart kadmind.
`service kadmin stop` typically sends SIGTERM, which is caught by
verto and passed to our handler. Besides cleanup, we then call
verto_break(), which causes the verto_run() event loop to return. The
weird return code has been present since the addition of the kadmin
code, which used a similar event model for signals.