Change select/cons_res logic for socket allocations

Patch from Martin Perry.

SelectType=select/cons_res
SelectTypeParameters=CR_Socket

Slurm built with ALLOCATE_FULL_SOCKET = 1

Node n8 has the following layout:
Socket 0: CPUs 0-3
Socket 1: CPUs 4-7

Without fix to _allocate_sockets (incorrect allocation for -c values of 3, 5, 6, and 7):

[sulu] (slurm) etc> srun -c1 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c2 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c3 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-3 Mem=0
[sulu] (slurm) etc> srun -c4 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c5 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-4 Mem=0
[sulu] (slurm) etc> srun -c6 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-5 Mem=0
[sulu] (slurm) etc> srun -c7 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-6 Mem=0
[sulu] (slurm) etc> srun -c8 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-7 Mem=0

With fix to _allocate_sockets (allocation appears correct for all values of -c):

[sulu] (slurm) etc> srun -c1 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c2 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c3 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c4 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=4-7 Mem=0
[sulu] (slurm) etc> srun -c5 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-7 Mem=0
[sulu] (slurm) etc> srun -c6 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-7 Mem=0
[sulu] (slurm) etc> srun -c7 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-7 Mem=0
[sulu] (slurm) etc> srun -c8 -m block:block --jobid 1 scontrol --details show job 1 | grep CPU_ID
     Nodes=n8 CPU_IDs=0-7 Mem=0
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