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/common.h

@@ -881,6 +881,7 @@ bool fs_is_directory(const std::string & path);
 
 std::string fs_get_cache_directory();
 std::string fs_get_cache_file(const std::string & filename);
+std::string fs_get_config_directory();
 
 struct common_file_info {
     std::string path;