ntor3, rend3: clarify extension field defaults.
These patch changes describe new default behaviors for extension field lists, as appear in ntor3 and in many places throughout the ntor3 protocol. In general:
- Unrecognized extensions MUST be ignored.
Additionally, all the following rules apply _unless otherwise stated in the documentation for an extension.
- Extensions are sent in sorted order.
- Extensions should only be sent once in a message
- If you receive multiple copies of an extension, only the first one counts.
This comes out of discussions on tor!525 (closed).