RFC: backporting GEOM to the 4.x branch

[Justin C. Sherrill]

> Speaking of examining code, do you guys use data flow diagrams
> and other visual representation systems so you can work more quickly
> and effectively on such a huge and complex code base?
>
> I'm using doxygen and graphviz and I have to say they're really great,
> as can be seen from the following g_bde_work struct example:

The subject of doxygen has come up a few times, and people haven't been
too keen on it:

http://leaf.dragonflybsd.org/mailarchive/kernel/2003-10/msg00197.html
http://leaf.dragonflybsd.org/mailarchive/kernel/2003-10/msg00199.html

I'm assuming you'd have to have the extra data from doxygen to generate
those graphs, as I've not done that process, myself.