Fix misleading error for immediate alloc requests and defer combination.

When an allocation request was done with the immediate=1 argument and
SchedulerParameters included defer, Slurm was returning a misleading
ESLURM_FRAGMENTATION error. Logic now a returns a more appropriate
ESLURM_CAN_NOT_START_IMMEDIATELY error for this scenario by decoupling
defer from the too fragmented logic in job_allocate().

Note that this doesn't change behavior as immediate + defer combination
continues having defer as the king in terms of precedence order, meaning
individual submit time allocation attempts will be avoided independently
of immediate.

Bug 5175.