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/zebra/zserv.c b/zebra/zserv.c
index dc3d432..d558b2d 100644
--- a/zebra/zserv.c
+++ b/zebra/zserv.c
@@ -1733,11 +1733,11 @@
 
 /* Make zebra server socket, wiping any existing one (see bug #403). */
 void
-zebra_zserv_socket_init (void)
+zebra_zserv_socket_init (char *path)
 {
 #ifdef HAVE_TCP_ZEBRA
   zebra_serv ();
 #else
-  zebra_serv_un (ZEBRA_SERV_PATH);
+  zebra_serv_un (path ? path : ZEBRA_SERV_PATH);
 #endif /* HAVE_TCP_ZEBRA */
 }