Add untrusted connection security infrastructure (v3)

Replace the v2 blacklist approach with a per-command flag system.
Commands must opt in to being available for untrusted connections
via flag_untrusted_safe (0x400), checked in call_command() which
catches all execution paths including nested commands.

Infrastructure changes:
- Add flag_untrusted_safe to CommandMap
- Add untrusted_error exception type for proper error codes
- Enforce trust check in both call_command() overloads
- Add catch blocks in xmlrpc_c, xmlrpc_tinyxml2, and jsonrpc handlers
- Port SCGI trust state management from v2 (thread_local, header parsing)
- Add _U macro variants in command_helpers.h for safe command registration
- Add CMD2_VAR_*_U and CMD2_VAR_*_U_GET variants for variables

src/rpc/command_map.h

@@ -56,6 +56,8 @@ public:
   static const int flag_file_target    = 0x100;
   static const int flag_tracker_target = 0x200;
 
+  static const int flag_untrusted_safe = 0x400;
+
   CommandMap() = default;
 
   bool                has(const std::string& key) const { return base_type::find(key) != base_type::end(); }