fix: netbsd_sysctl.persist with new key

When the key was not already present in sysctl.conf, the creatin was
still trying to access the (un)matched line groups.
fix: (netbsd)sysctl.persist (state was broken)

states/sysctl calls sysctl.persist with config file, regardless of the
OS. *_sysctl do have that parameter, but netbsd_sysctl.

Now, netbsd_sysctl takes a config file with a default value like the
others.
Merge pull request #18349 from rallytime/bp-18049

Backport #18049 to 2014.7