lib: Improve error reporting from broken config files

* command.h: (config_from_file) Add variable to interface for line
      number reporting.
    * command.c: (config_from_file) Set & increment 'line_num' while parsing.
    * vty.c: (vty_read_file) Report parse errors in the correct order to
      stderr, with added line numbers.
diff --git a/lib/command.c b/lib/command.c
index 7249c65..1087ceb 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -2762,13 +2762,15 @@
 
 /* Configration make from file. */
 int
-config_from_file (struct vty *vty, FILE *fp)
+config_from_file (struct vty *vty, FILE *fp, unsigned int *line_num)
 {
   int ret;
+  *line_num = 0;
   vector vline;
 
   while (fgets (vty->buf, VTY_BUFSIZ, fp))
     {
+      ++(*line_num);
       vline = cmd_make_strvec (vty->buf);
 
       /* In case of comment line */