Either apply the dynamic change of audit log name or don't allow the change when logging is ON

Quote from the testing comments:

> - changing setting server_audit_file_path dynamically is allowed, but has no effect;
that's how it's done. The variable is changed but the value is only used when the logging is started.

Well, it can't stay like that. People will set the path, and days later won't be able to find the log, or worse still, will remove the file with the old name, being sure it's unused.

There are options:
1) make it work "on fly". After all, it works for general log, so why not for audit plugin?
2) if it's impossible or undesirable on whatever reason, forbid changing the log name when server_audit_logging is ON, with a proper error message. It will provide clear information to the person who is trying to change the path, and (if the change is necessary) will make them restart the logging to ensure that the new name is indeed picked up.