Rename SLURM_DIST_HOSTFILE to SLURM_DIST_ARBITRARY to better describe the

function of this setting in the slurm protocol.

Eliminate redundant and incorrect "enum distribution_t" in env.h and replace
it with "enum task_dist_states" from slurm.h.  distribution_t was causing
the following inconsistency:

morrone:~$ srun -N3 -n6 -l env |grep SLURM_NODEID |sort
0: SLURM_NODEID=0
1: SLURM_NODEID=0
2: SLURM_NODEID=1
3: SLURM_NODEID=1
4: SLURM_NODEID=2
5: SLURM_NODEID=2
morrone:~$ srun -N3 -n6 -A
morrone:~$ srun -l env |grep SLURM_NODEID |sort
0: SLURM_NODEID=0
1: SLURM_NODEID=1
2: SLURM_NODEID=2
3: SLURM_NODEID=0
4: SLURM_NODEID=1
5: SLURM_NODEID=2