DAL cleanup

[ghudson@mit.edu]

We've known for a while that our database access vtable interface is
less than perfect.  As part of a mini-project of documenting the DAL
interface (which I'll check in by the end of Monday, barring
emergencies), I've recorded a list of identified problems and proposed
solutions.  It's at:

http://k5wiki.kerberos.org/wiki/Projects/Database_Access_Layer_cleanup

This is categorized as an early project page, but it's not really a
project yet, as there's been no determination of scope or resources or
time; that will come out of the process of choosing deliverables for
1.9.

The page has gotten kind of big, but people are welcome to read over
it and point out any red flags, or suggest additions.