Update macOS 7,8,9 comments to make it clear it refers to non-darwin based older MacOS

[plicease]

There are some references to "MacOS" in comments that are not OS X or later aware. These should be reworded so that it is clear that they are referring to pre-OS X. I've always used the term OS 9, but technically I think MacPerl could run on OS 7 or OS 8. The official terminology seems to be "classic environment" although I find this somewhat awkward. Perhaps the first time we say something like "OS9 here refers to the 'classic environment' and covers MacOS 7 and 8 as well" and use the shorter less awkward OS9 terminology in the rest of the source code.