Convert --network option parsing to new form.

Use slurm_option_set_by_env() in srun in one location, and remove
network_set_env from srun_opt_t.

Drop the direct SLURM_NETWORK envvar set in the srun option parsing -
this was duplicated further below for Cray systems (the only ones using
this variable) in _opt_args() and thus can be removed here.