Merge Windows wrappers into relevant functions. Remove more unnecessary unescaping. Merge *OpenW into *Open functions. Use unbuffered IO for output.
These functions were never declared in public headers, so it should be safe to hide them. Fixes #139.
Private functions were previously declared - in header files in the root directory - in public headers guarded with IN_LIBXML - in libxml.h - redundantly in source files that used them. Consolidate all private header files in include/private.
