Build add-ons redirector support on download.php script

Right now, installation and/or upgrading of add-ons is downloading stuff from moodle.org/plugins (MDL-38509, MDL-39403...)

While it's not planned to change that anytime soon, it was commented @ early stages of the development that it would be better to, always, point to some URL at download.moodle.org and this one would decide (redirect/serve...) the final URL.

We have such URL already, it's called https://download.moodle.org/download.php

And this is about to extend it, allowing "addons" so any request in the form:

http://download.moodle.org/download.php/addons/673/block_admin_presets_moodle22_2012032800.zip

is considered valid, accounted and, for now, redirected to:

http://moodle.org/plugins/download.php/673/block_admin_presets_moodle22_2012032800.zip

In the future, this can lead to caching/moving techniques in a transparent way for all the install/upgrade addons machinery.

Ciao