Add preboot script hook

[wgwoods]

There are some problems that have to be fixed before we reboot to start the upgrade; for example, replacing the KEYTABLE/KEYMAP boot argument with vconsole.keymap in Fedora 18.

fedup should supply a directory for such scripts to be run before reboot.

[wgwoods]

Alternately, we might be able to defer this to the packagers if we had some way of discovering/installing required updates before beginning the upgrade; see #42 for more on that.