Convert --hint option parsing to new form.

Note one important difference: this delays processing of --hint until
the verify pass, at which time --ntasks-per-core or --threads-per-core
may have been evaluated. Previously, order of argument evaluation would
change the result, despite a comment on each of these warning against
such behavior.

Also switch the order of SLURM_HINT and SBATCH_HINT - SBATCH_HINT will
now override SLURM_HINT if both are set.