Tor should not use purely-internal 0xF2 DNS response code

As discussed here !830 (comment 3094607) and in torspec#278 (closed), this error code ought not to have been assigned for this purposes.

Therefore the code in C Tor ought to be fixed to not use it.

Ideally, C Tor with this use of this error code hasn't yet been released. If so the error code could be put back into the unregistered pool.

Since this error code is a single byte, an easy fix for C Tor would be to use a bigger variable (int, say) and a value outside the range 0-255 for this special case. (I haven't looked at the C Tor code to see whether this would be easy.)

CC @dgoulet @DanWin