Introduced an entrypoint script to autotune the number of worker processes

The script is a no-op bye default, you would need to enable its logic by
setting an NGINX_ENTRYPOINT_WORKER_PROCESSES_AUTOTUNE variable to any
value.

The script then tries to get the following values:

 - getconf _NPROCESSORS_ONLN
 - the amount of cpus from cpuset cgroup
 - the quotas from cpu/cpuacct cgroup

The lowest of all three is then applied to nginx.conf.