mb_str_pad: multi-bytes str_pad implementation

Allows to pad an UTF-8 string.

Function signature is identical to str_pad with an extra $encoding
parameter:

mb_str_pad($input, $pad_length, $pad_string = ' ', $pad_type = STR_PAD_RIGHT, $encoding = mb_internal_encoding())

Initially written by Ronald Ulysses Swanson.