slashes at the end of cds tab completion

[aryarm]

cd will add slashes to the end of its suggestions. I want cds to do this too because then people won't have to type a slash each time they want to autocomplete another subdirectory.

[aryarm]

Things that will need to be done

• [x] add a trailing slash for all suggestions returned by our faked call to _cd
• [x] add a trailing slash for all suggestions involving completion of cds <letter(s)>[tab] but not cds + or cds -