torspec lies about accepting both IPv4 and IPv6 for ORAddress lines

(From this comment on #9380)

tl;dr: The "a"/"or-address" lines, in implementation, only happen once each per router, and only ever contain IPv6 addresses, despite what dir-spec.txt says.

The spec says:

     "a" SP address ":" port NL

        [Any number]

        The "or-address" element as specified in section 2.1.1.

and:

   "or-address" SP ADDRESS ":" PORT NL

       [Any number]
      
       ADDRESS = IP6ADDR | IP4ADDR
       IPV6ADDR = an ipv6 address, surrounded by square brackets.
       IPV4ADDR = an ipv4 address, represented as a dotted quad.
       PORT = a number between 1 and 65535 inclusive.                
       An alternative for the address and ORPort of the "router" line, but with
       two added capabilities:  
      
         * or-address can be either an IPv4 or IPv6 address
         * or-address allows for multiple ORPorts and addresses

       A descriptor SHOULD NOT include an or-address line that does nothing but
       duplicate the address:port pair from its "router" line.

       The ordering of or-address lines and their PORT entries matter because
       Tor MAY accept a limited number of addresses or ports. As of Tor 0.2.3.x
       only the first address and the first port are used.

• In terms of how many "a"/"or-address" lines there may be, the spec is only correct if you pay super close attention to the last sentence (this is actually the first time I've noticed it :) ).

• In terms of whether IPv4 and/or IPv6 addresses are acceptable, the spec is currently wrong, according to the functions router_rebuild_descriptor() [https://gitweb.torproject.org/tor.git/blob/9f9b19ed7b06d8313a9bcbd6647fa097ec0a059d:/src/or/router.c#l1811 [source]] and router_dump_router_to_string() [https://gitweb.torproject.org/tor.git/blob/9f9b19ed7b06d8313a9bcbd6647fa097ec0a059d:/src/or/router.c#l2330 [source]] in src/or/router.c in tor's source code.