bgpd: rework bgp_zebra_announce() function, separate nexthop handling

Separate the processing in bgp_zebra_announce(), by separating the
nexthop code in a separate function called
bgp_zebra_announce_parse_nexthop(). This commit does not bring any
functional change.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>