Properly handle references for nexthops and labels in filterset_copy()

Properly handle references for nexthops and labels in filterset_copy()

Up until recently filterset_copy() was only used in the parent process
where none of those references exist but by using filterset_copy() in
the RDE to populate per-peer output filters the code needs to handle
such references. Without this rules like 'match to any set nexthop X'
will cause internal corruption on config reloads and session resets.
Bug report from Marko Cupac (marko.cupac (at) mimar.rs)
OK tb@