Revert r37567 for nwo ("(bug 8604) padright: and similar functions fail with non...

Revert r37567 for nwo ("(bug 8604) padright: and similar functions fail with non-ASCII arguments")
This implements an mb_str_pad fallback function, but there is no mb_str_pad in PHP documentation, and the doc comments are really weird -- it says it returns an integer!
If this function is created from whole cloth and doesn't exist in PHP, it should be given a MediaWiki style name and not be done with a function_exists check as though it were a compat function.