[vtysh] Force line buffered mode.

Vtysh is used in pipelines and it helps if the output starts right
away instead of waiting for a whole buffer to fill.
diff --git a/vtysh/vtysh_main.c b/vtysh/vtysh_main.c
index 51f376c..d655e07 100644
--- a/vtysh/vtysh_main.c
+++ b/vtysh/vtysh_main.c
@@ -259,6 +259,7 @@
 
   /* Initialize user input buffer. */
   line_read = NULL;
+  setlinebuf(stdout);
 
   /* Signal and others. */
   vtysh_signal_init ();