From: Olof Johansson <olof@austin.ibm.com>

It's currently a boolean, but that means that system_running goes to zero
again when shutting down.  So we then use code (in the page allocator) which
is only designed to be used during bootup - it is marked __init.

So we need to be able to distinguish early boot state from late shutdown
state.  Rename system_running to system_state and give it the three
appropriate states.