Validate port options

Check `port` options for invalid values (ports are parsed as uint16, so
in practice values >65535 are invalid; port 0 is undefined and therefore
considered invalid too). This allows for an early rejection of faulty
values and an supplying an informative message to the user.

Splits tests in `feature_proxy.py` to cover both invalid `hostname`
and `port` values.

Adds a release-note as previously valid `-port` and `-rpcport` values
can now result in errors.

src/init.cpp

@@ -1255,6 +1255,51 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // as they would never get updated.
     if (!ignores_incoming_txs) node.fee_estimator = std::make_unique<CBlockPolicyEstimator>(FeeestPath(args));
 
+    // Check port numbers
+    for (const std::string port_option : {
+        "-port",
+        "-rpcport",
+    }) {
+        if (args.IsArgSet(port_option)) {
+            const std::string port = args.GetArg(port_option, "");
+            uint16_t n;
+            if (!ParseUInt16(port, &n) || n == 0) {
+                return InitError(InvalidPortErrMsg(port_option, port));
+            }
+        }
+    }
+
+    for (const std::string port_option : {
+        "-i2psam",
+        "-onion",
+        "-proxy",
+        "-rpcbind",
+        "-torcontrol",
+        "-whitebind",
+        "-zmqpubhashblock",
+        "-zmqpubhashtx",
+        "-zmqpubrawblock",
+        "-zmqpubrawtx",
+        "-zmqpubsequence",
+    }) {
+        for (const std::string& socket_addr : args.GetArgs(port_option)) {
+            std::string host_out;
+            uint16_t port_out{0};
+            if (!SplitHostPort(socket_addr, port_out, host_out)) {
+                return InitError(InvalidPortErrMsg(port_option, socket_addr));
+            }
+        }
+    }
+
+    for (const std::string& socket_addr : args.GetArgs("-bind")) {
+        std::string host_out;
+        uint16_t port_out{0};
+        std::string bind_socket_addr = socket_addr.substr(0, socket_addr.rfind('='));
+        if (!SplitHostPort(bind_socket_addr, port_out, host_out)) {
+            return InitError(InvalidPortErrMsg("-bind", socket_addr));
+        }
+    }
+
     // sanitize comments per BIP-0014, format user agent and check total size
     std::vector<std::string> uacomments;
     for (const std::string& cmt : args.GetArgs("-uacomment")) {