Add support for * width and precision in printf()

Add support for * width and precision in printf()

If * is used for width/precision in printf, then the width/precision
is provided by a printf argument instead of being part of the format
string. Semantics generally match those of printf in C.

This can be used to easily reproduce PHP's float printing behavior:

    // Locale-sensitive using precision ini setting.
    // Used prior to PHP 8.0.
    sprintf("%.*G", (int) ini_get('precision'), $float);

    // Locale-insensitive using precision ini setting.
    // Used since to PHP 8.0.
    sprintf("%.*H", (int) ini_get('precision'), $float);

    // Locale-insensitive using serialize_precision ini setting.
    // Used in serialize(), json_encode() etc.
    sprintf("%.*H", (int) ini_get('serialize_precision'), $float);

Closes GH-5432.