Add tmpl_cursor_next and tmpl_cursor_init which allow iterating over VALUE_PAIRs in the current request with a value_pair_tmpl_t

Rewrite tmpl_copy_vps and tmpl_find_vp to use the new cursor functions.

The iterators are significantly more useful than tmpl_copy_vps or tmpl_find_vp, but those functions
are still quite convenient.