shutdown:add message of sessioned on

description:
    when two users(A,B) have logged in,
    the user A run dde-shutdown,
    there should be a message to tell the user if shutdown or poweroff
    the computer,the programe running and some data of user B will lost!

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/410b8d2c1cc1412781bdb4431a916350/

Change-Id: I2bd9996598184d944733e9d3e24cbd59ae73a1fd