Update base-files.r
Update base-files.r

Now returns 'file or _ for `exists?` instead of always _

See https://github.com/metaeducation/ren-c/issues/519
draegtun/ren-c,kealist/ren-c,draegtun/ren-c,draegtun/ren-c,rgchris/ren-c,kealist/ren-c,rgchris/ren-c,hostilefork/rebol,rgchris/ren-c,kealist/ren-c,giuliolunati/ren-c,hostilefork/rebol,draegtun/ren-c,rgchris/ren-c,giuliolunati/ren-c,hostilefork/rebol,hostilefork/rebol,kealist/ren-c,draegtun/ren-c,kealist/ren-c,codebybrett/ren-c,codebybrett/ren-c,codebybrett/ren-c,giuliolunati/ren-c,kealist/ren-c,codebybrett/ren-c,codebybrett/ren-c,hostilefork/rebol,draegtun/ren-c,hostilefork/rebol,codebybrett/ren-c,rgchris/ren-c,giuliolunati/ren-c,giuliolunati/ren-c