mbstring: Fix prototypes

mb_convert_case(): 2nd parameter is not optional so default value is meaningless.
mb_decode_numericentity(): 3rd parameter is optional and its default value is internal encoding.
mb_encode_numericentity(): 3rd parameter is optional and its default value is internal encoding.
                           added 4th parameter as of PHP 5.4.0.
mb_regex_set_options(): no default value of "options"


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@329377 c90b9560-bf6c-de11-be94-00142212c4b1