Fix bug where running `boardgame-util serve` from a place with a folder that begins with 't' would have a problem. Reimplement path.RelativizePath. Part of #656.
Fix bug where running `boardgame-util serve` from a place with a folder that begins with 't' would have a problem. Reimplement path.RelativizePath. Part of #656.

jkomoros/boardgame,jkomoros/boardgame,jkomoros/boardgame