Fix v3.1 regression, segfault when inetd service connection exits

The service_step() function may call svc_del() on an svc from the
service_monitor(), so we need a way to tell the latter that it no
longer can dereference the pointer.

This regression is limited to v3.1, due to the introduction of the
new non-shared-memory store of svc_t.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/service.h

@@ -34,7 +34,7 @@ void      service_unregister     (svc_t *svc);
 void      service_runtask_clean  (void);
 void      service_reload_dynamic (void);
 
-void      service_step           (svc_t *svc);
+int       service_step           (svc_t *svc);
 void      service_step_all       (int types);
 
 void      service_bootstrap_cb   (uev_t *w, void *arg, int events);