bgpd: Fix memleak, adapt adv- to recv-routes code

bgpd: Fix memleak, adapt adv- to recv-routes code

This commit tries to adapt a similar codeflow within the `show bgp [afi]
[safi] neighbor <neighbor> advertised-routes` command compared to its
`received-routes` and `filtered-routes` opponents. Some branching code
has been restructured to achieve this.

Additionally, this commit fixes a memory leak within `received-routes`
(and `filtered-routes`, although the issue has been present before the
previous commit!) where the previous implementation forgot to
deduplicate the BGP attributes.

When a user called `<...> received-routes route-map <RM-TEST>` and that
routemap changed any AS path or community parameters, the duplicated
memory for these parameters was never freed. This has been fixed by
ensuring to call `bgp_attr_undup()` accordingly.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>