Replace num_nodes in allocation RPCs with min_nodes and max_nodes.

Also add exclude=node_list to allocate RPCs.
slurmctld accepts new RPC arguments, but does nothing with them (yet).
Besides processing new arguments, srun: will process node lists from
file rather than in-line list, sets SLURM_NNODES environment variable
based upon actual count of allocated nodes, and constraints info
in debug mode reported in full (was only reporting last constraint
found).