lib/vty: add vty_stdio()

this introduces a new public/API function to the vty code for opening a
VTY on stdin/stdout.  Intended for unrestricted use by the individual
daemons, i.e. "offical API".

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/vty.h b/lib/vty.h
index b758df3..abac3dd 100644
--- a/lib/vty.h
+++ b/lib/vty.h
@@ -237,6 +237,7 @@
 extern void vty_terminate (void);
 extern void vty_reset (void);
 extern struct vty *vty_new (void);
+extern struct vty *vty_stdio (void);
 extern int vty_out (struct vty *, const char *, ...) PRINTF_ATTRIBUTE(2, 3);
 extern void vty_read_config (char *, char *);
 extern void vty_time_print (struct vty *, int);