Add ability to tolerate (ignore) old config options

This allows us to explicitly ignore (with a warning) old options
that are no longer supported.

llarp/config/config.cpp

@@ -433,6 +433,7 @@ namespace llarp
 
       ConfigDefinition conf;
       initializeConfig(conf, params);
+      addBackwardsCompatibleConfigOptions(conf);
 
       ConfigParser parser;
       if (!parser.LoadFile(fname))
@@ -505,6 +506,23 @@ namespace llarp
     logging.defineConfigOptions(conf, params);
   }
 
+  void
+  Config::addBackwardsCompatibleConfigOptions(ConfigDefinition& conf)
+  {
+    auto addIgnoreOption = [&](const std::string& section, const std::string& name) {
+      conf.defineOption<std::string>(section, name, false, true, "", [=](std::string arg) {
+        (void)arg;
+        LogWarn("*** WARNING: The config option [", section, "]:", name, " is deprecated");
+      });
+    };
+
+    addIgnoreOption("system", "user");
+    addIgnoreOption("system", "group");
+    addIgnoreOption("system", "pidfile");
+
+    addIgnoreOption("api", "authkey");
+  }
+
   void
   ensureConfig(
       const fs::path& defaultDataDir, const fs::path& confFile, bool overwrite, bool asRouter)

llarp/config/config.hpp

@@ -185,6 +185,13 @@ namespace llarp
     void
     initializeConfig(ConfigDefinition& conf, const ConfigGenParameters& params);
 
+    /// Insert config entries for backwards-compatibility (e.g. so that the config system will
+    /// tolerate old values that are no longer accepted)
+    ///
+    /// @param conf is the config to modify
+    void
+    addBackwardsCompatibleConfigOptions(ConfigDefinition& conf);
+
     // Load a config from the given file
     bool
     Load(const char* fname, bool isRelay, fs::path defaultDataDir);