avoid using _ in scripts and file names to steer around the difference

[masinter]

i'd prefer eliminating _ in scripts whenever possible rather than introducing references to

             XCL-USER::EXEC_INTERLISP Apps.AroundExitFn)
             XCL-USER::EXEC�INTERLISP Apps.AroundExitFn)

because the appearance in Medley and in text editors.

Originally posted by @masinter in #2474 (review)

[masinter]

closing this issue. I would prefer to remove the possibility of errors where there is a risk of using the wrong external-format by just avoiding the characters in things like script names and so on, but this is just a preference.

If we ever make a "LispCore" primer for system developers, this preference might be noted there.