[xml] Character encodng cleanup

["Peter Jacobi" <pj walter-graphtek com>]

Hi Daniel, All,

As already discussed, using the xmlCharEncoding enum must go
away, as the enum is always in danger of not covering an exotic
encoding.

So far, work proceeds relatively painless, but look into the
usage pattern in xmlIO.c: 

xmlAllocParserInputBuffer and related functions pass around
the current encoding as xmlCharEncoding enum, so two
questions arise:

1) Must these versions of the functions be kept for API
compatiBility? 

I fear yes, it's a major PITA, that libxml2 make
nearly every internal detail visible?

2) Should the replacement functions pass the encoding
as encoding name or as xmlCharEncodingHandlerPtr?

As other functions in xmlIO.c have decided to use
xmlCharEncodingHandlerPtr, I would prefer following
this trend.

Regards,
Peter Jacobi