Allow audit plugin to specify timestamp format when writing to file

If writing to a file, the audit plugin has a fixed timestamp format which does not include a timezone.
This means that should the audit be centralised, unless the database's server is running in UTC time, one must use syslog to gain a timestamp that can include timezone information.

It is suggested a new server audit system variable is defined, which if set, contains a string the strftime() routine applies with it's format argument.