Clean up handling of various memory options.

Change mem_per_cpu / mem_per_gpu / pn_min_memory / pn_min_tmp_disk
to uint64_t in slurm_opt_t, and use NO_VAL64 as the sentinel value
throughout.

Rework str_to_mbytes2() to return a uint64_t, and NO_VAL64 as an invalid
value instead of -1.