Fixing directory-fold to return knil on non-existing directories.
Fixing file-regular?/directory?/... shortcut tests to return #f
for non-existent files instead of a confusing error.
Adding the -R<module> option to run "main" procedures directly from modules,
analogous to Python's -m.  With no argument, runs chibi.repl.
Don't warn on import redefining an undefined value.