Added code to provide a preliminary single user mode.

Added code to provide a preliminary single user mode.
CtdlTrySingleUser() - Try to go single user
CtdlEndSingleUser() - End single user
CtdlIsSingleUser() - True is system is single user
CtdlWantSingleUser() - True if system is trying to go single user

The system is said to be in single user mode when there is only one
session active. Which should be the one that asked it to go single user.

When trying to go single user new connections are refused as per the
nologin mechanism and scheduled threads are prevented.