Windows: Use a customized struct stat that also has the st_blocks member.

Windows: Use a customized struct stat that also has the st_blocks member.

Windows's struct stat does not have a st_blocks member. Since we already
have our own stat/lstat/fstat implementations, we can just as well use
a customized struct stat. This patch introduces just that, and also fills
in the st_blocks member. On the other hand, we don't provide members that
are never used.

Signed-off-by: Johannes Sixt <redacted>