o Merge defaults and requirements.
o Get rid of get_default_sysctl_value().
o Manually set 'type' for e.g. boot enviroment tunables.
o Cache sysctl map once per boot.
o Edit system defaults for easier override.
While sysctls might change when (un)loading kernel modules the
risk of missing something vital is not given. We could always
flush the cache file in that case later.