Directory-opening never worked on Windows because MSVCRT's open()
doesn't open directories and its fstat() doesn't recognise directory
handles. These are just MSVCRT restrictions, and the Windows API itself
has no problem with opening directories as file objects, so reimplement
mpv's mp_open and mp_stat to use the Windows API directly. This should
fix directory playback.
This also populates the st_dev and st_ino fields of struct stat, so
filesystem loop checking in demux_playlist.c should now work on Windows.
Fixes#4711