MINOR: cpu-topo: rely on _SC_NPROCESSORS_CONF to trim maxcpus

MINOR: cpu-topo: rely on _SC_NPROCESSORS_CONF to trim maxcpus

We don't want to constantly deal with as many CPUs as a cpuset can hold,
so let's first try to trim the value to what the system claims to support
via _SC_NPROCESSORS_CONF. It is obviously still subject to the limit of
the cpuset size though. The value is stored globally so that we can
reuse it elsewhere after initialization.