ddk/ntifs.h: merge reactos.org r55151 (RtlUnicodeToMultiByteN() UnicodeString
parameter is PCWCH type, not PWCH), and r59376 changes (correct some FSCTL_*
definitions.)

git-svn-id: svn://svn.code.sf.net/p/mingw-w64/code/trunk@5926 4407c894-4637-0410-b4f5-ada5f102cad1