core: when restarting services, don't close fds

We would close all the stored fds in service_release_resources(), which of
course broke the whole concept of storing fds over service restart.

Fixes #4408.

src/core/unit.c

@@ -302,6 +302,7 @@ int unit_set_description(Unit *u, const char *description) {
 
 bool unit_check_gc(Unit *u) {
         UnitActiveState state;
+        bool inactive;
         assert(u);
 
         if (u->job)
@@ -311,16 +312,17 @@ bool unit_check_gc(Unit *u) {
                 return true;
 
         state = unit_active_state(u);
+        inactive = state == UNIT_INACTIVE;
 
         /* If the unit is inactive and failed and no job is queued for
          * it, then release its runtime resources */
         if (UNIT_IS_INACTIVE_OR_FAILED(state) &&
             UNIT_VTABLE(u)->release_resources)
-                UNIT_VTABLE(u)->release_resources(u);
+                UNIT_VTABLE(u)->release_resources(u, inactive);
 
         /* But we keep the unit object around for longer when it is
          * referenced or configured to not be gc'ed */
-        if (state != UNIT_INACTIVE)
+        if (!inactive)
                 return true;
 
         if (u->no_gc)