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

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 every time 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.

This pull requests makes all options used in /etc/init/xbmc.conf configurable in the defaults file (/etc/default/xbmc).
While at it I've removed some duplicated code from the `postinst` script: options no longer need to be defined there too, as they're extracted from /var/tmp/xbmc.