v4: Further simplifications and fixes for tmpl_dcursors (#4651)

v4: Further simplifications and fixes for tmpl_dcursors (#4651)

* Amalgamate child_eval functions for all tmpl types

* Remove unused list_head from eval function

* Define _tmpl_cursor_common_push

to clarify push / pop on the nested cursor stack

* Move matching of attribute da to dcursor iterator function

* Just use a single child cursor init function

The only difference between a leaf and intermediate node is allocation
of the #tmpl_cursor_nested_t

* Evaluation function no longer needed