Upgrade System Replacement

The pfSense upgrade system is going to be rewritten in HEAD and RELENG_1.

The way it'll work is the user goes to the upgrade page, it polls something giving a list of what version(s), if any, are available for
upgrade. Click the desired one, and it downloads and installs the upgrade.

Offline upgrades are also a must-have, so the existing upgrade functionality will likely be retained as is (though improvements are up for discussion).

cvsup and rsync are two options being discussed for the basis of a new upgrade system.

A two stage upgrade process is under consideration, with one stage upgrading the OS, and one for the pfSense code (web UI, startup scripts, etc.).