Split out GlobalConfig into its own header

This makes it easier to understand the reach of global variables /
global state in the config system.

src/libutil/config-global.cc

@@ -0,0 +1,66 @@
+#include "config-global.hh"
+
+namespace nix {
+
+bool GlobalConfig::set(const std::string & name, const std::string & value)
+{
+    for (auto & config : *configRegistrations)
+        if (config->set(name, value)) return true;
+
+    unknownSettings.emplace(name, value);
+
+    return false;
+}
+
+void GlobalConfig::getSettings(std::map<std::string, SettingInfo> & res, bool overriddenOnly)
+{
+    for (auto & config : *configRegistrations)
+        config->getSettings(res, overriddenOnly);
+}
+
+void GlobalConfig::resetOverridden()
+{
+    for (auto & config : *configRegistrations)
+        config->resetOverridden();
+}
+
+nlohmann::json GlobalConfig::toJSON()
+{
+    auto res = nlohmann::json::object();
+    for (const auto & config : *configRegistrations)
+        res.update(config->toJSON());
+    return res;
+}
+
+std::string GlobalConfig::toKeyValue()
+{
+    std::string res;
+    std::map<std::string, Config::SettingInfo> settings;
+    globalConfig.getSettings(settings);
+    for (const auto & s : settings)
+        res += fmt("%s = %s\n", s.first, s.second.value);
+    return res;
+}
+
+void GlobalConfig::convertToArgs(Args & args, const std::string & category)
+{
+    for (auto & config : *configRegistrations)
+        config->convertToArgs(args, category);
+}
+
+GlobalConfig globalConfig;
+
+GlobalConfig::ConfigRegistrations * GlobalConfig::configRegistrations;
+
+GlobalConfig::Register::Register(Config * config)
+{
+    if (!configRegistrations)
+        configRegistrations = new ConfigRegistrations;
+    configRegistrations->emplace_back(config);
+}
+
+ExperimentalFeatureSettings experimentalFeatureSettings;
+
+static GlobalConfig::Register rSettings(&experimentalFeatureSettings);
+
+}