service: flush status text and errno values each time a service is started

We shouldn't show status texts from previous service starts

(cherry picked from commit 8cfdb077b8)

src/core/service.c

@@ -1699,6 +1699,9 @@ static int service_start(Unit *u) {
         s->main_pid_alien = false;
         s->forbid_restart = false;
 
+        free(s->status_text);
+        s->status_text = NULL;
+
         service_enter_start_pre(s);
         return 0;
 }