fixed compilation with latest LZMA SDK on Windows.

LZMA SDK recently added an #include <windows.h> to its headers, meaning it's no longer safe to include its headers globally in platform independent files.
The following changes were necessary:

- rename DWORD type in zipdir.c
- add USE_WINDOWS_DWORD and reorder includes in file_7z.cpp
- wrap LZMA decoder stream into a local struct that's declared anonymously in files.h and adjust files.cpp for this change.