dcs: sort of implement DCS passthrough

We now store the passthrough characters in a buffer, and call
dcs_passthrough() on unhook.

However, dcs_passthrough() doesn't do anything.

main.c

@@ -819,6 +819,7 @@ out:
         xkb_context_unref(term.kbd.xkb);
 
     free(term.vt.osc.data);
+    free(term.vt.dcs.data);
     for (int row = 0; row < term.normal.num_rows; row++)
         grid_row_free(term.normal.rows[row]);
     free(term.normal.rows);