Exit with status 0 from kadmind

Exit with status 0 from kadmind

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.

ticket: 8650 (new)