Use mount and umount syscalls when handling cgroup namespaces.

Using the syscalls directly rather than calling bin/(u)mount via
system() avoids a few fork + exec calls, and provides better error
handling if something goes wrong.

Users of this functionality are also updated to use slurm_strerror in
order to provide a more informative error message.

The mount and umount syscalls are Linux-specific, but so are cgroups
so no portability is lost.