Fix #8313: use correct capitalization for TTO / DOS base-music (#8385)

DOS did not have cases in filenames. Different OS interpret
them as either all-lowercase or all-uppercase. So we try both.
All-uppercase is done by the obg/obm/obs files, and if opening
fails, OpenTTD will automatically retry the all-lowercase variant.

So for those who already have the files lowercase, nothing
changes. For those that install fresh from TTO, it should now
work out-of-the-box.