app: audit: Pad the microseconds out in log_login()

In log_login() we log a timestamp in the form seconds.microseconds since
the epoch in the utmp table.

This was being done with the following format string

    %ld.%ld

Change this to

    %ld.%06ld

So 1356000609.19697 would now be stored as 1356000609.019697

This matches the rest of the timestamps.

Signed-off-by: Andrew Clayton <andrew@opentechlabs.co.uk>