Always enable `core-file`

Currently to enable core dumps these need to be enabled on the MariaDB server side, and the OS side, too.

I'd request to either have `core-file` enabled by default, or maybe even have it deprecated and eventually removed, having it "always on" on the MariaDB server side.

Downside would be that systems with classic "just write core dumps to local files" may suffer from the disk space used, but usually such files are severely limited in size, or even disabled by default; so default impact should be low.

And especially on systems with systemD core dump service and `coredumpctl` available and enabled (the majority by now?) size used by core dump is not an issue as the coredump collection service takes care of rotation / purging automatically.

Alternatively: only have `core-file` enabled by default when knowing that the server is running under control of a systemD service, regardless of actual configuration setting? Or just change the default to "ON" in this case.