lib/vty: add separate output fd support to VTYs

to be used with stdin/stdout terminals, this adds support for writing to
a different FD than we're reading from.  Also fixes error messages from
config load being written to stdin.

[v2: fixed config write]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/command.c b/lib/command.c
index 8ae27de..3a59f24 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -3065,7 +3065,7 @@
   
   /* Make vty for configuration file. */
   file_vty = vty_new ();
-  file_vty->fd = fd;
+  file_vty->wfd = fd;
   file_vty->type = VTY_FILE;
 
   /* Config file header print. */