Refactor drush_pm to standardize terminology and simplify update logic

Drush_pm uses very confusing variable names, mixing up projects/releases/packages and using meaningless names such as data and info in ways that make the contents of the variable very hard to determine without tracing each one back to it's source. This patch lays down a standard terminology and naming convention across the main install and update functions.

In addition, it also does the following:
* Splits the mammoth 130 line drush_pm_update() function into 2 - one that does the listing of module status, and prepares the array of updates to be made, and another to confirm and actually do the updates.
* Adds various inline comments to explain the intention
* Reorders some logic to group related operations (e.g. user output) closer together.
* Removing unrequested updates now comes after the table display - so you always see a list of all available updates (rather than just your selected updates) - this is a change in behavior, but now actually matches the description of the table ("Update status information on all installed and enabled Drupal modules").
* The output before the summary of modules that are about to be updated has been tweaked to make it clear that these *will* be updated (the previous output didn't actually explain what would happen if you hit "y").
* Renamed drush_pm_install_package() to drush_pm_install(), to make it in line with drush_pm_update() and also remove the inconsistent singular 'package' (when it accepts multiples).

Finally, this patch adds the ability to update to a specific version of a module using the project-1.x-dev type command, without needing to remove and reinstall it as you do now. This is unrelated to the refactoring, but I decided to leave it in for now since it is now quite straightforward (literally just the "Preprocess releases" loop and the 2 new constants/case options). If anyone wants however, I am happy to split this out and we can review separately at http://drupal.org/node/238762.

Drupal 5 and 6 patches attached. I have tested them both and they seem pretty solid to me - it would be worth someone else giving them a run over before we commit however.