Propagate real_memory_size to slurmstepd at job start

Add conf->real_memory_size to the list of slurmd_conf_t members that
are propagated to slurmstepd during a job step launch. This makes the
amount of RAM available on the system (as determined by slurmd) available
for use in slurmstepd plugins or slurmstepd itself, without having to
recalculate its value.