Regenerating docs and API files
Various cleanups
* configure.in: force regeneration of APIs in my environment
* buf.c buf.h enc.h encoding.c include/libxml/tree.h
include/libxml/xmlerror.h save.h tree.c: various comment cleanups
pointed by apibuild
* doc/apibuild.py: added the 3 new internal headers in the excludes
* doc/libxml2-api.xml doc/libxml2-refs.xml: regenerated the API
* doc/symbols.xml: listing new entry points for 2.9.0
* doc/devhelp/*: regenerated
Adding new encoding function to deal with the new structures
* encoding.c: adds xmlCharEncFirstLineInput, xmlCharEncInput and
xmlCharEncOutput
* enc.h: the functions are not made public but added to this new header
