Change GNU to @acronym{GNU} in a few places.

Change GNU to @acronym{GNU} in a few places.
Use "set-user-ID" and "set-group-ID" a bit more consistently.
Use "appropriate privileges" rather than "super-user" a bit
more consistently.
(install invocation): Parent directories are now 755 without uid
or gid changing.  The default mode is now 0755, not 755.
(mkdir invocation): Rewrite the top-level usage description, since
I couldn't easily follow the old one.  It's now 3 lines not 8.
For -m, describe file permission bits versus other bits, and note
that mkdir is atomic if you don't mention special bits.
(chmod invocation): Mention what chmod does to setgid and setuid bits.