ntdll: Avoid one of the calls to getattrlist() when read_directory_getattrlist() checks the case sensitivity of the file system.

Extract a new function, get_dir_case_sensitivity_attr_by_id(), from
get_dir_case_sensitivity_attr().  Call that instead of get_dir_case_sensitivity()
from read_directory_getattrlist().  We get the device and file system IDs from
the same getattrlist() call we're already doing, so that avoids an extra call.

This ensures that the cookie is preserved when the server sets an additional cookie.

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