bgpd: When advertising a multipath route, the attribute set to be

bgpd: When advertising a multipath route, the attribute set to be
advertised is based on the bestpath attribute set, but the
following attributes are aggregated from the attribute sets
of the multipath constituents:
- AS_PATH
- ORIGIN
- COMMUNITIES
- EXTENDED COMMUNITIES

In addition the route is advertised with the NEXT_HOP set
to the router's interface IP address, instead of the NEXT_HOP
of the best path. This is to ensure that traffic will go to this
router so it can be fanned out via the multipath route.

* bgpd/ecommunity.c
  * ecommunity_uniq_sort(): Make this function externally accessible
* bgpd/ecommunity.h
  * Add external declaration for ecommunity_uniq_sort()
* bgpd/bgp_mpath.c
  * bgp_info_nexthop_cmp(): Replace calls to bgp_attr_extra_get()
    to avoid unwanted memory allocation
  * bgp_info_mpath_free(): Free aggregate attribute for multipath
  * bgp_info_mpath_attr(): Lookup aggregate attribute of a multipath route
  * bgp_info_mpath_attr_set(): Set aggregate attribute of a multipath route
  * bgp_info_mpath_aggregate_update(): Update the aggregate attribute
    of a multipath route
* bgpd/bgp_mpath.h
  * bgp_info_mpath: Add pointer to hold aggregate attribute of a multipath
  * Add external declarations for new functions
* bgpd/bgp_route.c
  * bgp_announce_check(): Use aggregate attribute when announcing multipath
    route
  * bgp_announce_check_rsclient(): Use aggregate attribute when announcing
    multipath route
  * bgp_best_selection(): After updating multipath set, update the
    multipath aggregate attribute