Change :if-*-exists default to :supersede
The difference between :overwrite (the former default) and :supersede is only that the latter should not lead to an error if the file does not exist. There is plenty of support for :supersede behaviour (not necessarily under that name) but not for :overwrite behaviour. Hence we make it the default and add a normaliser. Since normalisation is currently only necessary for CLISP and CLISP only has :if-output-exists support (not :if-error-output-exists or :if-input-does-not-exist), we only translate :if-output-exists for now.
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