This allows to report the reason why opening a file failed to the parser
context and improve error messages. Now we can also remove the stat call
before opening a file.
Don't create subcontext in xmlXIncludeRecurseDoc. Save and restore 'doc'
and 'incTab' instead.
Make xmlXIncludeLoadFallback call xmlXIncludeCopyNode which seems safer
than xmlXIncludeDoProcess since the latter may modify the document.
This should also be more performant since we need to copy the whole
fallback subtree anyway. Also make sure to avoid replacements in
fallback elements in xmlXIncludeDoProcess.
