Convert --threads in srun to new form.

Move warning about excessive values for --threads into arg_set_max_threads,
but soften the language since the limit has not been enforced for a while.

Do not reset this each pass. Especially since the only effect this option
has is to bump pmi_server_max_threads() around, and that doesn't not know
about HetJobs.

Move MAX_THREADS to slurm_opt.h, and rename to SRUN_MAX_THREADS to avoid
collision with the same macro name elsewhere. This should probably be made
into a configuration value at some point.