lib: log source of vty connections (bug #566)

heasley: I've used this for a while to track usage.
diff --git a/lib/vty.c b/lib/vty.c
index 30a94e1..e4818eb 100644
--- a/lib/vty.c
+++ b/lib/vty.c
@@ -1692,6 +1692,7 @@
   int accept_sock;
   struct prefix *p = NULL;
   struct access_list *acl = NULL;
+  char *bufp;
 
   accept_sock = THREAD_FD (thread);
 
@@ -1764,6 +1765,11 @@
     zlog (NULL, LOG_INFO, "can't set sockopt to vty_sock : %s", 
 	  safe_strerror (errno));
 
+  zlog (NULL, LOG_INFO, "Vty connection from %s",
+    (bufp = sockunion_su2str (&su)));
+  if (bufp)
+    XFREE (MTYPE_TMP, bufp);
+
   vty = vty_create (vty_sock, &su);
 
   return 0;