LOG_MODE_RE (used in validate_log_mode() in aamode.py) just checked if
the given parameter contains one of the possible matches. This resulted
in "invalid" being a valid log mode (from audit.log requested_mask or
denied_mask) because it contains 'a', which is a valid file mode.
This patch wraps the regex into ^(...)+$ to make sure the full
string contains only allowed file modes.
The patch also adds some tests for validate_log_mode().
Acked-by: Steve Beattie <steve@nxnw.org>