Audit: NULL values for output_type and file_path are exessive

(null) values for server_audit_file_path and server_audit_output_type make no sense: they are not needed to disable the logging, server_audit_logging takes care of that. (null) values only make configuration more complicated.

I suggest just to pick default values and set the variables to them, much like it's done for general log, slow log etc. I think server_audit_output_type=file would be more universal, and server_audit.log looks like a good candidate for the default log name.

bzr version-info

revision-id: holyfoot@askmonty.org-20130705160101-ni574i35exft3wn5

revno: 3825

branch-nick: 5.5-noga-hf