ntdll: Fix read_directory_getattrlist() to get the name of a symlink rather than its target, but still detect if the symlink is broken.

ntdll: If read_directory_getattrlist() finds that the file doesn't exist, return successful "no file" result to stop search.

The docs for getattrlist() say that ATTR_CMN_NAME can return up to NAME_MAX + 1
Unicode characters (code points) encoded in UTF-8, which means it can be up to
NAME_MAX * 3 + 1 bytes.

Based on a patch by Qian Hong.

These are from MSVC 2013.

It's necessary to return the actual filename with correct casing and a
plain stat doesn't allow that. Make read_directory_stat() return the
result of the stat() call on a case-insensitive filesystem only when the
file is missing.

When searching for a specific filename on a case-insensitive filesystem
we first try with stat(). If stat() does find the file we currently
return the requested filename back.

That presents an issue when the application cares about the casing of the
actual file stored on-disk. Specifically, NtQueryDirectoryFile is
supposed to return the actual filename with correct casing.

One possible solution to the issue, without having to resort to manually
scanning the directory entries, is to make use of the OS X getattrlist()
function, since it can return the filename stored on the filesystem.