Add support for * width and precision in printf()

[nikic]

Adds support for * width and precision, following glibc semantics.

Together with #5436 it allows easily printing floats exactly as PHP would:

// Locale-sensitive using precision ini setting.
sprintf("%.*G", ini_get('precision'), $float);

// Locale-insensitive using serialize_precision ini setting.
sprintf("%.*H", ini_get('serialize_precision'), $float);

[carusogabriel]

@nikic Shall we close https://bugs.php.net/bug.php?id=60755?

[nikic]

@carusogabriel Thanks for the reminder, done!