Merge bitcoin/bitcoin#27071: Handle CJDNS from LookupSubNet()

0e6f6ebc064c5fb425fc3699efe760ec6cd4b6af net: remove unused CConnman::FindNode(const CSubNet&) (Vasil Dimov)
9482cb780fe04c1f1d9050edd1b8e549e52c86ce netbase: possibly change the result of LookupSubNet() to CJDNS (Vasil Dimov)
53afa68026ffa1313ae4aba3664de7791d23b1c8 net: move MaybeFlipIPv6toCJDNS() from net to netbase (Vasil Dimov)
6e308651c441cbf8763c67cc099c538c333c2872 net: move IsReachable() code to netbase and encapsulate it (Vasil Dimov)
c42ded3d9bda8b273780a4a81490bbf1b9e9c261 fuzz: ConsumeNetAddr(): avoid IPv6 addresses that look like CJDNS (Vasil Dimov)
64d6f77907afd461d9b14ee10ab32335f4454734 net: put CJDNS prefix byte in a constant (Vasil Dimov)

Pull request description:

  `LookupSubNet()` would treat addresses that start with `fc` as IPv6 even if `-cjdnsreachable` is set. This creates the following problems where it is called:

  * `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).

  * `RPCConsole::unbanSelectedNode()`: in the GUI the ban entries go through `CSubNet::ToString()` and back via `LookupSubNet()`. Then it must match whatever is stored in `BanMan`, otherwise it is impossible to unban via the GUI.

  These were uncovered by https://github.com/bitcoin/bitcoin/pull/26859.

  Thus, flip the result of `LookupSubNet()` to CJDNS if the network base address starts with `fc` and `-cjdnsreachable` is set. Since subnetting/masking does not make sense for CJDNS (the address is "random" bytes, like Tor and I2P, there is no hierarchy) treat `fc.../mask` as an invalid `CSubNet`.

  To achieve that, `MaybeFlipIPv6toCJDNS()` has to be moved from `net` to `netbase` and thus also `IsReachable()`. In the process of moving `IsReachable()`, `SetReachable()` and `vfLimited[]` encapsulate those in a class.

ACKs for top commit:
  jonatack:
    Code review ACK 0e6f6ebc064c5fb425fc3699efe760ec6cd4b6af
  achow101:
    ACK 0e6f6ebc064c5fb425fc3699efe760ec6cd4b6af
  mzumsande:
    re-ACK 0e6f6ebc064c5fb425fc3699efe760ec6cd4b6af

Tree-SHA512: 4767a60dc882916de4c8b110ce8de208ff3f58daaa0b560e6547d72e604d07c4157e72cf98b237228310fc05c0a3922f446674492e2ba02e990a272d288bd566

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