Make dustmod migrate stats0 file when stats1 does not exist

[msg555]

We already load stats0 at https://github.com/msg555/df-script-src/blob/v22.7.1/source/game.hpp#L149 when stats1 does not exist. However we also need to logically migrate the data when this happens. Dustmaker now has logic to do this migration which would need to be replicated within Dustmod, see https://github.com/msg555/dustmaker/blob/v1.2.0/dustmaker/cmd/save_convert.py#L103.

To make this possible we would need to add the legacy fields back into stats_persist struct. I don't think t here's any conflict on struct key names/semantics so this should be no issue.