Clean up shmall and shmmax handling

Instead of overspecializing how we set postgres-related sysctl values, just set shmall and shmmax directly only if we need to, and record the settings we used in /etc/sysctl.d/ for posterity.  

This will work on all distros from approx. RHEL 6.x on.  If we want RHEL 5.x support as well, that can be added fairly trivially.