We now use /usr/local/opnsense/version/xxx as our version database.
Apart from the core package, opnsense-update(8) also puts its own
version information in there.  To stay somewhat consistent, the
version file should resemble the package name, even though that
looks strange in our case: /usr/local/opnsense/version/opnsense.
It's just that /usr/local/etc is too open and might clash with
other things in the future.