Switch to unsigned type for fr_lst_index_t (#4190)

Switch to unsigned type for fr_lst_index_t (#4190)

* Switch to unsigned type for fr_lst_index_t

This involves adding one to the true index when storing it in an
item, and subtracting one when reading it for use.

* Switch to unsigned type for fr_lst_index_t

This involves adding one to the true index when storing it in an
item, and subtracting one when reading it for use.

* Switch to unsigned type for fr_lst_index_t

This involves adding one to the true index when storing it in an
item, and subtracting one when reading it for use.