minor fixes

Hi,

In iTALC 1.0 the machine running the master interface would avoid running a demo or screenlock, even when it's in the selected set. Having this in iTALC 2.0 would be nice for us because not every lab has a dedicated teacher machine, and sometimes teachers sit at different machines. The proposed check and its comment are basically ported from 1.0, maybe a more elegant solution is possible. (Maybe disabling more commands too? These are the important ones since they take over the screen.)

The other change in this set is to update the session commands for GNOME 3. Probably the KDE 3 dcop commands could be dropped too, KDE 3 is long gone from most systems...

Thanks for considering these patches!