DIRSUFFIX and FILESUFFIX are strings added on unambiguous or exact completion.
This mirrors the optional behavior of tcsh."
:type '(choice (const :tag "None" nil)
- (const :tag "add /" t)
+ (const :tag "Add /" t)
(cons :tag "Suffix pair"
(string :tag "Directory suffix")
(string :tag "File suffix")))