Make link specifier handling in rend-spec-v3 more precise

Split off legacy/trac#26627 (moved).

We should specify that clients and services must not check untrusted link specifiers against the consensus: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n1338 https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n1705

Services should also copy unrecognized rend point link specifiers from the introduce cell to the rendezvous join cell. We can copy the text from the service intro->rend spec: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n1705 To the the client desc->intro spec: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n1338

Thanks to catalyst for picking up on these missing parts of the spec.

Edit: fix line numbers