core: allow to set exit status when using SuccessAction=/FailureAction=exit in units

This adds SuccessActionExitStatus= and FailureActionExitStatus= that may
be used to configure the exit status to propagate in when
SuccessAction=exit or FailureAction=exit is used.

When not specified let's also propagate the exit status of the main
process we fork off for the unit.

src/shared/bus-unit-util.c

@@ -1555,6 +1555,25 @@ static int bus_append_unit_property(sd_bus_message *m, const char *field, const
 
                 return bus_append_safe_atou(m, field, eq);
 
+        if (STR_IN_SET(field, "SuccessActionExitStatus", "FailureActionExitStatus")) {
+
+                if (isempty(eq))
+                        r = sd_bus_message_append(m, "(sv)", field, "i", -1);
+                else {
+                        uint8_t u;
+
+                        r = safe_atou8(eq, &u);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse %s=%s", field, eq);
+
+                        r = sd_bus_message_append(m, "(sv)", field, "i", (int) u);
+                }
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                return 1;
+        }
+
         if (unit_dependency_from_string(field) >= 0 ||
             STR_IN_SET(field, "Documentation", "RequiresMountsFor"))