spank: Update spank man page

Update spank(8) man page with documentation of new functionality,
including new callbacks: slurm_spank_slurmd_init, slurm_spank_slurmd_exit,
slurm_spank_job_prolog, and slurm_spank_job_epilog, as well as the new
spank_option_getopt() call for use in option processing by plugins.