Read defaults while starting too

I use a modified version of [this script](http://forum.xbmc.org/showthread.php?tid=115023) to add "_HOLD" events to my remote.

That script reads from the lircd socket and outputs to a different one, so everytime I update xbian-package-xbmc-scripts I need to modify the original xbmc.conf and change the line `env DAEMON_LIRC="/run/lirc/lircd"` to point to the proxied one.

By reading /etc/default/xbmc one can change these options without having to edit any /etc/init file.
Remove duplicate code from postinst.

Options no longer need to be defined there too, as these are extracted from /var/tmp/xbmc.

Support for "advanced options" (the ones used in /etc/init/xbmc.conf, but not configurable until now): these are just appended to the new defaults file.
Merge pull request #2 from anaconda/patch-1

Make all options configurable in /etc/default/xbmc, remove duplicated code.