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/scgi_task.h

@@ -53,6 +53,7 @@ private:
   unsigned int        m_buffer_size{0};
 
   ContentType         m_content_type{ XML };
+  bool                m_trusted{true};
 };
 
 }