Split task_dist_states variable

The task_dist_states variable has been split into "flags" and "base"
components. Added SLURM_DIST_PACK_NODES and SLURM_DIST_NO_PACK_NODES values
to give user greater control over task distribution. The srun --dist options
has been modified to accept a "Pack" and "NoPack" option. These options can
be used to override the CR_PACK_NODE configuration option.