Improve Dynamic CPU policy:
- add support for 'conservative' CPU governor
- when running on AC, try the following CPU governors (in this order):
ondemand, conservative, userspace
- when on battery, try the following CPU governors (in this order):
conservative, ondemand, userspace
<a href="http://launchpad.net/bugs/105259">http://launchpad.net/bugs/105259</a>;

Fix crash in case that unsupported CPU frequency governor is used. CPU
policies listed in menu are now dynamically added depending on support in HW.
<a href="http://launchpad.net/bugs/112120">http://launchpad.net/bugs/112120</a>;