Add SystemInfo::numAvailableCores

SystemInfo::numCores is less useful than might be desired, because many forms of process management can limit access to a subset of total cores. This causes us to scale certain internal subsystems at suboptimal levels by assuming more processing than we actually have available.

It would useful to have a new value available, perhaps numAvailableCores, which would return a num <= ::numCores.

Consider the implementation of gnu nproc and investigate if a portable solution exists.