Add new operations `os/file-directory?', `os/make-filename', and

Add new operations `os/file-directory?', `os/make-filename', and
`os/filename-as-directory'.  These operations replicate the action of
existing operations, but they allow the performance of filename
completion to be improved significantly over what could previously be
achieved.

Add new operation `os/completion-ignored-extensions' which is used to
initialize the variable of that name.

Change `os/directory-list-completions' to special case the null-string
prefix for added performance.