Break up packed job user messages to prepend index.

Was dumping this:
$ srun -A test7.21-account.1 --qos test7.21-qos.1 -n5 : -n3 : -n1 /bin/true
srun: error: 0: submit1
srun: error: submit2
srun: error: submit3
srun: error: Unable to allocate resources: Invalid account or account/partition combination specified

Will now dump this:
$ srun -A test7.21-account.1 --qos test7.21-qos.1 -n5 : -n3 : -n1 /bin/true
srun: error: 0: initialized
srun: error: 0: submit1
srun: error: 0: submit2
srun: error: 0: submit3
srun: error: Unable to allocate resources: Invalid account or account/partition combination specified

Bug 6513.