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.h

@@ -59,6 +59,10 @@ struct common_preset_context {
     bool filter_allowed_keys = false;
     std::set<std::string> allowed_keys;
 
+    // if true, options unknown to the current example are skipped instead of being an error
+    // used for config files shared by all binaries, where each binary only knows a subset of options
+    bool ignore_unknown_keys = false;
+
     // if only_remote_allowed is true, only accept whitelisted keys
     common_preset_context(llama_example ex);