[development] Eliminate hooks entirely in Drupal 6

[Earl Miles]

Dries Buytaert wrote:
> Talking about terminology; this is also referred to as 'listeners'.   
> The word 'callback' somehow /feels/ more generic than /listeners/.

When I think of listeners, I envision something that gets onto a well-named 
interface and then awaits instruction. Basic socket listeners, for example, 
which is where I know the word from, in fact.

The main difference I can think of with a callback is a callback is generally 
specifically registered to one client or server, and a listener tends to be 
open to multiple clients. But I dunno what the official definitions might be, 
that's just a gut feeling I get from various things I've worked with over time.