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.
Make "src" the implicit "/usr/local" so that nobody can
sneak in weird files in the base file system or potentially
overwrite them.
This is the last time, I swear... :)
