vtysh: fix function prototypes

This makes a whole bunch of vtysh functions static, fixes prototypes for
a few more, and masks user_free() and user_write_config() (both unused.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/vtysh/vtysh_user.h b/vtysh/vtysh_user.h
index 8d0a4cf..c485c23 100644
--- a/vtysh/vtysh_user.h
+++ b/vtysh/vtysh_user.h
@@ -22,6 +22,7 @@
 #ifndef _VTYSH_USER_H
 #define _VTYSH_USER_H
 
-int vtysh_auth ();
+int vtysh_auth (void);
+void vtysh_user_init (void);
 
 #endif /* _VTYSH_USER_H */