system: shutdown/restart process for krang computer and script should respect other users

[cerdogan]

When multiple people are plugged in and somebody does not realize others have ssh'ed to Krang computer and perform a restart, it can interrupt other's work. We should write a script to wrap the "shutdown" command, to check for other users.

[saulrh]

We can use the w, who, and wall commands to handle the logic and broadcasting.