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/lib/zclient.h b/lib/zclient.h
index 21786ab..5c3db38 100644
--- a/lib/zclient.h
+++ b/lib/zclient.h
@@ -127,11 +127,8 @@
 extern void zclient_reset (struct zclient *);
 extern void zclient_free (struct zclient *);
 
-/* Get TCP socket connection to zebra daemon at loopback address. */
-extern int zclient_socket (void);
-
-/* Get unix stream socket connection to zebra daemon at given path. */
-extern int zclient_socket_un (const char *);
+extern int  zclient_socket_connect (struct zclient *);
+extern void zclient_serv_path_set  (char *path);
 
 /* Send redistribute command to zebra daemon. Do not update zclient state. */
 extern int zebra_redistribute_send (int command, struct zclient *, int type);