config: add [allowed_keys] allow_all=yes by default, add [gui] as valid section

- node_config.cpp: add [allowed_keys] allow_all=yes to default config
- config_parser.c: add SECTION_GUI to suppress 'Key outside section' warnings

tools/chatgui/transport/node_config.cpp

@@ -345,6 +345,9 @@ bool NodeConfig::save() {
     out << "debug_level=" << (m_debugLevel.isEmpty() ? "info" : m_debugLevel) << "\n";
     out << "debug_categories=" << m_debugCategories << "\n\n";
 
+    out << "[allowed_keys]\n";
+    out << "allow_all=yes\n\n";
+
     out << "[ntp]\n";
     out << "enabled=yes\n";
     out << "server=pool.ntp.org\n";