task/cgroup: Don't create memory cgroups with limit of 0 bytes

Treat a 0 byte memory limit from SLURM as unlimited and instead use
MaxRAMPercent and MaxSwapPercent as RAM and Swap limits for the job/job
step. This avoids creating a memory cgroup with limit_in_bytes = 0,
which would end up causing the cgroup to OOM before slurmstepd could
even be started.

This also allows systems in which SLURM isn't explicitly allocating
memory to use the task/cgroup plugin with ConstrainRAMSpace=yes.