bgpd: Pass global ASN for confederation peers if not AS_SPECIFIED

bgpd: Pass global ASN for confederation peers if not AS_SPECIFIED

When we specify remote-as as external/internal, we need to set local_as to
bgp->as, instead of bgp->confed_id. Before this patch, (bgp->as != *as) is
always valid for such a case because *as is always 0.

Also, append peer->local_as as CONFED_SEQ to avoid other side withdrawing
the routes due to confederation own AS received and/or malformed as-path.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit db5a5ee6e4665b5f951ed2a37398820da4b762f1)