fix mandoc warnings

- sort references in SEE ALSO section to section number, name.
- cols.1: proper order of sections.
- wrap lines to max 79 column length.

du.1

@@ -15,9 +15,11 @@
 .Nm
 displays the file system block usage for each
 .Ar file
-argument and for each directory in the file hierarchy rooted in directory argument.
-If no file is specified, the block usage of the hierarchy rooted in the current
-directory is displayed.
+argument and for each directory in the file hierarchy rooted in directory
+argument. If no
+.Ar file
+is specified, the block usage of the hierarchy rooted in the current directory
+is displayed.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl a