Detecting the cpus based on sysconf of the online CPUs can significantly
over estimate the number of cpus available.

Wheither via numactl, cgroups, taskset, systemd constraints, docker
containers and probably other mechanisms, the number of threads mysqld
can be run on can be quite less.

As such we use the pthread_getaffinity_np function on Linux and FreeBSD
(identical API) to get the number of CPUs.

The number of CPUs is the default for the thread_pool_size and a too
high default will resulting in large memory usage and high context
switching overhead.

Closes PR #922