arg: use CSV escape style for multiple-value args

[ngxson]

Also fix #18635 along the way

Example usage:

argv = {"binary_name", "--lora", "file1.gguf,\"file2,2.gguf\",\"file3\"\"3\"\".gguf\",file4\".gguf"};
assert(true == common_params_parse(argv.size(), list_str_to_char(argv).data(), params, LLAMA_EXAMPLE_COMMON));
assert(params.lora_adapters.size() == 4);
assert(params.lora_adapters[0].path == "file1.gguf");
assert(params.lora_adapters[1].path == "file2,2.gguf");
assert(params.lora_adapters[2].path == "file3\"3\".gguf");
assert(params.lora_adapters[3].path == "file4\".gguf");

[ServeurpersoCom]

Looks good to me! I test it on a server + it need a doc update:)
The minimalist CSV parser is acceptable for this use case. Edge cases (unclosed quotes, mid-field quotes) produce malformed paths but won't crash, the file-not-found error downstream will catch them anyway. (and no memory safety issues)