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.

Drop relative_set in favor of direct test against NO_VAL.
Zero is also a valid value to have set this to - change two tests
to only check against NO_VAL.

Note that this argument's behavior is different from --wait in sbatch.

Drop --preserve-slurm-env as an alias; this has not been documented
since at least version 2.6.

Drop the call to uname() as the name variable is unused.

Move default setting of slurm_get_srun_epilog() to _opt_verify().

Move default setting of slurm_get_srun_prolog() to _opt_verify().

Needed to get rid of the last option in _opt_default(). Add an
internal slurm_opt_umask without a corresponding CLI option to
handle the translation for us.

Need to send these options through slurm_process_option() for correct
tracking, so make it easy to translate to Slurm's argument value and
pass along.

Move job_name handling down alongside similar code in _opt_verify().

Move setting of SLURM_EXPORT_ENV in sbatch to _opt_verify().

Note some structural work to support this: add a new _env_early_opt()
to parse out key early environment variables, so that xlate_batch_script()
is no longer directly parsing the cli arguments or looking directly
for SBATCH_IGNORE_PBS in the environment.