Handling case where opendir fails in directory-fold-tree.
If unbound, using current-output-port in place of current-error-port for -x languages.
Allows -xscheme.r5rs.
Adding a shortcut for -xscheme.base.
Fixing failed opendir handling from 1725:739f90999b17.
Adding introspection to available unloaded modules.
Adding missing changes to available module introspection from 1729:f1e5f235d684.
Adding closedir call to directory-fold-tree.
Adding repl-advise-exception, currently advising potential modules exporting unbound indentifiers.