Convert --input in sbatch to new form.

Move ifname to slurm_opt_t, rather than keeping it in sbatch_opt_t
and srun_opt_t, making the arg handling code much simpler.

Skip setting it to /dev/null as part of the reset, and move that logic
to _opt_verify().

Fix a mistake in the API in slurm_get_job_stdin() where StdIn= is
prepended to /dev/null if ifname == NULL.