Dynamic attributes are now declared in per-protocol grammar files instead

of filter/config.Y. Bird now compiles even if you disable RIP.

Removed RTA and IMPOSSIBLE tokens (unused).

Removed superfluous comment in filter.h.

I've tried to do my best, but Pavel, please check these changes.

filter/f-util.c

@@ -32,6 +32,15 @@ f_new_inst(void)
   return ret;
 }
 
+struct f_inst *
+f_new_dynamic_attr(int code)
+{
+  struct f_inst *f = f_new_inst();
+  f->aux = T_INT;
+  f->a2.i = code;
+  return f;
+}
+
 char *
 filter_name(struct filter *filter)
 {