Fix nonsensical -norpcbind and -norpcallowip behavior

Treat specifying -norpcbind and -norpcallowip the same as not specifying
-rpcbind or -rpcallowip, instead of failing to bind to localhost and failing to
show warnings.

Also add code comment to clarify what intent of existing code is.

Author: ryanofsky

src/httpserver.cpp

@@ -362,16 +362,20 @@ static bool HTTPBindAddresses(struct evhttp* http)
     std::vector<std::pair<std::string, uint16_t>> endpoints;
 
     // Determine what addresses to bind to
-    if (!(gArgs.IsArgSet("-rpcallowip") && gArgs.IsArgSet("-rpcbind"))) { // Default to loopback if not allowing external IPs
+    // To prevent misconfiguration and accidental exposure of the RPC
+    // interface, require -rpcallowip and -rpcbind to both be specified
+    // together. If either is missing, ignore both values, bind to localhost
+    // instead, and log warnings.
+    if (gArgs.GetArgs("-rpcallowip").empty() || gArgs.GetArgs("-rpcbind").empty()) { // Default to loopback if not allowing external IPs
         endpoints.emplace_back("::1", http_port);
         endpoints.emplace_back("127.0.0.1", http_port);
-        if (gArgs.IsArgSet("-rpcallowip")) {
+        if (!gArgs.GetArgs("-rpcallowip").empty()) {
             LogPrintf("WARNING: option -rpcallowip was specified without -rpcbind; this doesn't usually make sense\n");
         }
-        if (gArgs.IsArgSet("-rpcbind")) {
+        if (!gArgs.GetArgs("-rpcbind").empty()) {
             LogPrintf("WARNING: option -rpcbind was ignored because -rpcallowip was not specified, refusing to allow everyone to connect\n");
         }
-    } else if (gArgs.IsArgSet("-rpcbind")) { // Specific bind address
+    } else { // Specific bind addresses
         for (const std::string& strRPCBind : gArgs.GetArgs("-rpcbind")) {
             uint16_t port{http_port};
             std::string host;