Fix #8713: Change OTTD2FS and FS2OTTD to return string objects #8716
Motivation / Problem
Change the functions to return
Additionally, this removes mostly any pretense that we can still build Windows 95 versions of the game.
Several of these changes are done blind, so far. Only tested in a single configuration on Windows, the changes for other OS may not compile.
There are unrelated changes here that should probably be split off.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.