Merge bitcoin/bitcoin#25989: init: abort if i2p/cjdns are chosen via -onlynet but are unreachable

68209a7b5c rpc: make addpeeraddress work with cjdns addresses (Martin Zumsande)
a8a9ed67cc init: Abort if i2p/cjdns are chosen via -onlynet but unreachable (Martin Zumsande)

Pull request description:

  If the networks i2p / cjdns are chosen via `-onlynet` but the user forgot to provide `-i2psam` / `-cjdnsreachable`, no outbound connections will be made - it would be nice to inform the user about that.
  The solution proposed here mimics existing behavior for `-onlynet=onion` and non-specified `-onion`/`-proxy` where we already abort with an InitError - if reviewers would prefer to just print a warning, please say so.

  The second commit adds CJDNS support to the debug-only `addpeeraddress` RPC allowing to add CJDNS addresses to addrman for testing and debug purposes. (if `-cjdnsreachable=1`)

  This is the result of an [IRC discussion](https://bitcoin-irc.chaincode.com/bitcoin-core-dev/2022-09-01#848066;) with vasild.

ACKs for top commit:
  vasild:
    ACK 68209a7b5c
  dergoegge:
    ACK 68209a7b5c

Tree-SHA512: 6db9787f01820190f14f90a0b39e4206603421eb7521f792879094d8bbf4d4d0bfd70665eadcc40994ac7941a15ab5a8d65c4779fba5634c0e6fa66eb0972b8d

src/init.cpp

@@ -1285,6 +1285,11 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }
 
     if (!args.IsArgSet("-cjdnsreachable")) {
+        if (args.IsArgSet("-onlynet") && IsReachable(NET_CJDNS)) {
+            return InitError(
+                _("Outbound connections restricted to CJDNS (-onlynet=cjdns) but "
+                  "-cjdnsreachable is not provided"));
+        }
         SetReachable(NET_CJDNS, false);
     }
     // Now IsReachable(NET_CJDNS) is true if:
@@ -1757,6 +1762,11 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
         SetProxy(NET_I2P, Proxy{addr});
     } else {
+        if (args.IsArgSet("-onlynet") && IsReachable(NET_I2P)) {
+            return InitError(
+                _("Outbound connections restricted to i2p (-onlynet=i2p) but "
+                  "-i2psam is not provided"));
+        }
         SetReachable(NET_I2P, false);
     }