(ITS#6987) Consider only same attribute siblings for Cft_misc.

For all other ce_types it holds that each is represented by one
attribute, making ce_type matching for X-ORDERED siblings equivalent to
how back-ldif handles them. Cft_misc is means "all other types", however
no overlay/backend so far has used different attributes as siblings,
triggering the difference in handling between bconfig.c and back-ldif.