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 every time `xbian-package-xbmc-scripts` is updated 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 on start one can change these options without having to edit any /etc/init file.