Another version of stacker.c (URL attached)

dwheelerat_private

Here's yet another version of stacker.c.  Thanks for the feedback, folks!
Rather than post a mailbomb, I've placed the current version at:
 http://www.dwheeler.com/misc/stacker.c

I've added a date - the current version is dated 2002-07-20, which
is (proudly?) displayed near the beginning of the file.
I'll just update this URL with newer versions, since I don't expect
anyone to be interested in old drafts of this thing yet.

Changes:
* More documentation, especially noting that modules that want to be
  stackable must NOT, when stacked, try to do another's job.
  E.G., in capable(), your code should look like this:
        if (!secondary)
                if (cap_is_fs_cap (cap) ? tsk->fsuid == 0 : tsk->euid == 0)
                         return 0;
        return -EPERM;
* Also noted that modules that want to be stackable need to set
  "secondary" to 1 _before_ they register as a child module, to avoid
  a subtle race condition that only shows up when you try to write
  a stacking module.  This is an interesting race condition that is
  universal among stacking modules.  It means, BTW, that stackable
  modules (like owlsm) will need to have their secondary = 1
  statement moved.  Details in the comments.
* Implemented a number of sys_security calls that apply to the
  stacker itself.  This includes the ability to disable adding children,
  removing children, and removing the module itself.
* Added support for removing children.
* Fixed a race condition when stacker is being stacked underneath
  ANOTHER multiplexor - now set secondary before registering.

Greg K-H has made a few really nice comments that I haven't figured
out how to respond to (re: locking and do .. while(0)).
For the moment, they're in the comments as TODO items so they won't get lost
or forgotten.

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