quagga: option "-z" ("--socket <path>") added

All daemons modified to support custom path to zserv
socket.

lib: generalize a zclient connection

zclient_socket_connect added. zclient_socket and
zclient_socket_un were hidden under static expression.
"zclient_serv_path_set" modified.
diff --git a/bgpd/bgp_nexthop.c b/bgpd/bgp_nexthop.c
index 719cb96..60efa19 100644
--- a/bgpd/bgp_nexthop.c
+++ b/bgpd/bgp_nexthop.c
@@ -1098,12 +1098,7 @@
   if (zlookup->sock != -1)
     return 0;
 
-#ifdef HAVE_TCP_ZEBRA
-  zlookup->sock = zclient_socket ();
-#else
-  zlookup->sock = zclient_socket_un (ZEBRA_SERV_PATH);
-#endif /* HAVE_TCP_ZEBRA */
-  if (zlookup->sock < 0)
+  if (zclient_socket_connect (zlookup) < 0)
     return -1;
 
   return 0;