Disable job expansion by default.

Add new SchedulerParameter of permit_job_expansion to permit expansion of
running jobs.

Replace the two calls to select_g_job_expand_allow() as all plugins return
true anyways at this point. Return ESLURM_NOT_SUPPORTED instead of
ESLURM_DEPENDENCY in one location as well to provide a better error
message back to the user.

Bug 5716.