netbase: possibly change the result of LookupSubNet() to CJDNS

All callers of `LookupSubNet()` need the result to be of CJDNS type if
`-cjdnsreachable` is set and the address begins with `fc`:

* `NetWhitelistPermissions::TryParse()`: otherwise `-whitelist=` fails
  to white list CJDNS addresses: when a CJDNS peer connects to us, it
  will be matched against IPv6 `fc...` subnet and the match will never
  succeed.

* `BanMapFromJson()`: CJDNS bans are stored as just IPv6 addresses in
  `banlist.json`. Upon reading from disk they have to be converted back
  to CJDNS, otherwise, after restart, a ban entry like (`fc00::1`, IPv6)
  would not match a peer (`fc00::1`, CJDNS).

* `setban()` (in `rpc/net.cpp`): otherwise `setban fc.../mask add` would
  add an IPv6 entry to BanMan. Subnetting does not make sense for CJDNS
  addresses, thus treat `fc.../mask` as invalid `CSubNet`. The result of
  `LookupHost()` has to be converted for the case of banning a single
  host.

* `InitHTTPAllowList()`: not necessary since before this change
  `-rpcallowip=fc...` would match IPv6 subnets against IPv6 peers even
  if they started with `fc`. But because it is necessary for the above,
  `HTTPRequest::GetPeer()` also has to be adjusted to return CJDNS peer,
  so that now CJDNS peers are matched against CJDNS subnets.

src/httpserver.cpp

@@ -682,7 +682,7 @@ CService HTTPRequest::GetPeer() const
         evhttp_connection_get_peer(con, (char**)&address, &port);
 #endif // HAVE_EVHTTP_CONNECTION_GET_PEER_CONST_CHAR
 
-        peer = LookupNumeric(address, port);
+        peer = MaybeFlipIPv6toCJDNS(LookupNumeric(address, port));
     }
     return peer;
 }