fix of idle nodes cannot be allocated

avoid add/remove node resource of job if the node is lost by resize

 I found another case that idle node can not be allocated. It can be
reproduced as follows:

1. run a job with -k option:

    [root@mn0 ~]# srun -w cn[18-28] -k sleep 1000
    srun: error: Node failure on cn28
    srun: error: Node failure on cn28
    srun: error: cn28: task 10: Killed
    ^Csrun: interrupt (one more within 1 sec to abort)
    srun: tasks 0-9: running
    srun: task 10: exited abnormally
    ^Csrun: sending Ctrl-C to job 106120.0
    srun: Job step aborted: Waiting up to 2 seconds for job step to
finish.

2. set a node down and then set it idle:

    [root@mn0 ~]# scontrol update nodename=cn28 state=down reason="hjcao
test"
    [root@mn0 ~]# scontrol update nodename=cn28 state=idle

3. restart slurmctld

    [root@mn0 ~]# service slurm restart
    stopping slurmctld:                                        [  OK  ]
    slurmctld is stopped
    starting slurmctld:                                        [  OK  ]

4. cancel the job

then, the node set down will be left unavailable:

    [root@mn0 ~]# sinfo -n cn[18-28]
    PARTITION AVAIL  TIMELIMIT  NODES  STATE NODELIST
    work*        up   infinite     11   idle cn[18-28]

    [root@mn0 ~]# srun -w cn[18-28] hostname
    srun: job 106122 queued and waiting for resources

    [root@mn0 slurm]# grep cn28 slurmctld.log
    [2013-03-18T15:28:02+08:00] debug3: cons_res: _vns: node cn28 in
exclusive use
    [2013-03-18T15:29:02+08:00] debug3: cons_res: _vns: node cn28 in
exclusive use

I made an attempt to fix this by the attached patch. Please review it.
Note nature of latest change