Changes handling of AS_PATH_CONFED_* segments in AS_PATH.

Although standard says that if we receive AS_PATH_CONFED_*
(and we are not a part of a confederation) segment, we should
drop session, nobody does that and it is unwise to do that.

Now we drop session just in case that peer ASN is in
AS_PATH_CONFED_* segment (to detect peer that considers BIRD
as a part of its confederation).