Added RPC for slurmctld generated shutdown RPC. This SHUTDOWN_IMMEDIATE

RPC is used to distinguish it from the SHUTDOWN RPC comming from scontrol.
It was added to deal with a race condition.
Kevin's change in where pthread_create occurs and how the RPC is
processed is now the default (#ifdef kevins_way removed).
Added limit on the thread count before going to non-threaded RPCs
(see MAX_THREAD_COUNT in controller.c).