lib: do not reopen a zclient socket for bfd

lib: do not reopen a zclient socket for bfd

b7ca809d1c ("lib: BFD automatic source selection") has added a dedicated
zclient socket for nht tracking. Since the bfd lib is used by daemons
that already has a zclient socket, those daemons has now a second
zclient socket. However, zebra does not distinguish the two zclient
sessions. For example, the interfaces are asked a second via
zebra_message_send(zclient, ZEBRA_INTERFACE_ADD, VRF_DEFAULT) in
zclient_start(). As a result, callbacks functions like bgp_ifp_create()
are called a second time, which causes some processing overhead and
might cause bugs.

Re-use the existing zclient socket for nht tracking.

Note that BFD automatic source selection is only currently implemented
in staticd. Other daemons will require to add the following in their
ZEBRA_NEXTHOP_UPDATE callback function:

> 	if (zclient->bfd_integration)
>		bfd_nht_update(&matched, &nhr);

Fixes: b7ca809d1c ("lib: BFD automatic source selection")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>