common: add system-level config file (#26118)

* common: Add CLI > ENV > models-presets > INI precedence

1. CLI flags have the highest precedence
2. ENV vars have the second-highest precedence
3. System and User configs have the lowest precedence
   - Linux/BSD/Mac
     - /etc/llama.cpp/config.ini < ${XDG_CONFIG_HOME:-~/.config}/llama.cpp/config.ini
   - Windows
     - %PROGRAMDATA%\llama.cpp\config.ini < %APPDATA%\llama.cpp\config.ini

* fix UB

* use common_get_env

* ignore_unknown_keys

* nits

* add docs

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

common/preset.cpp

@@ -322,6 +322,8 @@ common_presets common_preset_context::load_from_ini(const std::string & path, co
                     preset.options[opt] = value;
                 }
                 LOG_DBG("accepted option: %s = %s\n", key.c_str(), preset.options[opt].c_str());
+            } else if (ignore_unknown_keys) {
+                LOG_WRN("ignoring option '%s' from %s: not supported by this program\n", key.c_str(), path.c_str());
             } else {
                 throw std::runtime_error(string_format(
                     "option '%s' not recognized in preset '%s'",