[development] multiple menus- close to working in HEAD

[Peter Wolanin]

For those not following the issue queue closely, this could use some
feedback and review:
http://drupal.org/node/137767

It should probably be marked "critical".  Without a patch like this,
there is not good support in HEAD for multiple menus (e.g. "primary
links" in addition to  "navigation").

I would particularly like feedback on the last patch[1] that proposes
a mechanism for tracking the path to the root of the menu tree that
should make the system fast.  It's also pretty easy to understand the
algorithm (good for maintainability), but it has the minor drawback of
requiring that we set an arbitrary limit on the depth of the menu
tree.  Note that the existing menu system in HEAD essentially has
already a limit of 6 levels deep.

To put these patches in context, the goal is to be able to support the
successor to the book module (i.e. a new hierarchical page structure
module per Dries' wish list and the writeup I did as a SoC suggestion
[2]).  So, imagine a {menu_links} table with ~10,000 nodes links in it
(i.e. the drupal.org handbook pages).  Note that in a quick search I
can't find anywhere in the d.o handbooks that more than 4 levels deep.

-Peter

[1]: http://drupal.org/node/137767#comment-240315
[2]: http://drupal.org/node/128731