Fix problems relating to handler encoding

[dmlloyd]

• Handlers previously were initialized with a null encoding; this PR causes the encoding to always be initialized to the platform default
• Keep a Charset for all ExtHandlers to use so that they do not have to look up the encoding by name; replace the JUL encoding methods with versions that operate on the Charset
• Don't render head/tail by default if they do not transcode to the target encoding (for Unicode art banners)