[util] Set stdio files to line buffering

So we can stream lines to hb-shape and read output.

util/options.cc

@@ -780,6 +780,8 @@ text_options_t::get_line (unsigned int *len)
     gs = g_string_new (nullptr);
   }
 
+  setlinebuf (fp);
+
   g_string_set_size (gs, 0);
   char buf[BUFSIZ];
   while (fgets (buf, sizeof (buf), fp)) {
@@ -817,6 +819,8 @@ output_options_t::get_file_handle (void)
     fail (false, "Cannot open output file `%s': %s",
 	  g_filename_display_name (output_file), strerror (errno));
 
+  setlinebuf (fp);
+
   return fp;
 }