This will be useful when converting content on the fly on write or read -
allowing for support for output filter for file storage backend.

This is necessary for some mail clients which indicate encoding in GB2312
but actually use GBK encoding.

Also fix bug where bogus charset names like `iso` were confused with valid
charset names starting with that text, such as `iso-8859-2`.

This pull request adds a cleanup util for bogus and invalid charsets, mostly
added by a nameless company out of Redmond, WA.