bgpd: Implement BGP confederation error handling (RFC5065, Par. 5)

This patch implements BGP confederation error handling in Quagga as described
in RFC5065, paragraph 5.

* bgp_aspath.c: (aspath_confed_check, aspath_left_confed_check) new functions
* bgp_attr.c: (bgp_attr_aspath_check) apply previous and NOTIFY if there's
  a problem.
diff --git a/bgpd/bgp_aspath.h b/bgpd/bgp_aspath.h
index 2b4625c..9854d18 100644
--- a/bgpd/bgp_aspath.h
+++ b/bgpd/bgp_aspath.h
@@ -88,6 +88,8 @@
 extern int aspath_loop_check (struct aspath *, as_t);
 extern int aspath_private_as_check (struct aspath *);
 extern int aspath_firstas_check (struct aspath *, as_t);
+extern int aspath_confed_check (struct aspath *);
+extern int aspath_left_confed_check (struct aspath *);
 extern unsigned long aspath_count (void);
 extern unsigned int aspath_count_hops (struct aspath *);
 extern unsigned int aspath_count_confeds (struct aspath *);