Fix documentation and implementation of 'directory-name-p'

* lisp/files.el (directory-name-p): Modify to recognize
backslashes on MS-Windows and MS-DOS.  Adjust the doc string
accordingly.  Use '=', not char-equal, for comparison, as
letter-case cannot possibly be an issue here.

* doc/lispref/files.texi (Directory Names): Move the documentation
of directory-name-p here from "Relative File Names".  Update the
description per the changes in implementation.

* etc/NEWS: Move the entry for 'directory-name-p' to its proper
place and mark it documented.

1 files changed, 8 insertions, 3 deletions

diff --git a/lisp/files.el b/lisp/files.el
index f37c23b7bdd..d20bb1401fa 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -740,9 +740,14 @@ The path separator is colon in GNU and GNU-like systems."
        (error "No such directory found via CDPATH environment variable"))))
 
 (defsubst directory-name-p (name)
-  "Return non-nil if NAME ends with a slash character."
-  (and (> (length name) 0)
-       (char-equal (aref name (1- (length name))) ?/)))
+  "Return non-nil if NAME ends with a directory separator character."
+  (let ((len (length name))
+        (lastc ?.))
+    (if (> len 0)
+        (setq lastc (aref name (1- len))))
+    (or (= lastc ?/)
+        (and (memq system-type '(windows-nt ms-dos))
+             (= lastc ?\\)))))
 
 (defun directory-files-recursively (dir regexp &optional include-directories)
   "Return list of all files under DIR that have file names matching REGEXP.