[Bug 56647] ntdll-Junction_Points prevent rustup from correctly installing a toolchain

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=56647

--- Comment #4 from Lorenzo Ferrillo  ---
Eric the issue here is that these folders aren't symlinks, they are unix
mountpoints, and readlink doesn't work for them. 
IO_REPARSE_TAG_MOUNT_POINT is setted only if S_ISDIR is true and the folder
device is different then the parent device, or their inode are the same. The
function for that is fd_is_mount_point and pre-exist the ntdll_Junction-Points
patchset however they weren't actually setted without implementing
FILE_OPEN_REPARSE_POINT usage.