Use cinnamon instead of /usr/bin/cinnamon in session files

This allows running cinnamon in a different prefix like /usr/local,
provided the bin folder in that prefix is included in $PATH.

cinnamon2d is not changed, since the cinnamon2d executable is always in the same location: it's from the files/usr/bin directory, so always /usr/bin/cinnamon2d and not /usr/local/bin/cinnamon2d.