Make it possible to disable overclock and overvolt when XBMC is idle.

**Needs xbianonpi/xbian-update#3 merged**.

This is implemented as a configuration option in /etc/default/xbmc.
Set SCR_OVERCLOCK_OFF=yes to disable overclocking and overvolting when the screensaver starts.  The CPUFreq governor gets changed to "conservative".
When the screensaver is stopped it's changed back to the default CPUFreq governor specified in /etc/default/xbian-module-init-tools ("performance" by default).

The screensaver CPUFreq governor can be configured with SCR_CPUFREQ_GOV, e.g., SCR_CPUFREQ_GOV="powersave".

Currently lacks XBian-Config support.