ls: add option to reverse the sort order

Now you can list directory contents ordered by mtime with the
latest file displayed last in the output.

ls.1

@@ -24,6 +24,9 @@ print the index number of each file.
 lists detailed information about each file, including their type, permissions,
 links, owner, group, size, and modification time.
 .TP
+.B \-r
+reverses the sort order.
+.TP
 .B \-t
 sorts files by modification time instead of by name.
 .TP