Exec optimization

In some deployments with many sysctl options it takes some time to apply all sysctl options (in our case up to 60 secs) because of two execs for each option:  "sysctl -w" and "sysctl -n". Instead of this its might be effective to reload sysctl only once, after writing all options to the files. The check if some sysctl option is configured is dropped, if you want to have it I can create another type/provide to have "sysctl -a" with instances/prefetch.

And another small change is to allow usage of other files then /etc/sysctl.conf, like /etc/sysctl.d/config.conf