audit: include subject in login records

The login uid change record does not include the selinux context of the
task logging in.  Add that information.

(Updated from 2011-01: RHBZ:670328 -- RGB)

Reported-by: Steve Grubb <sgrubb@redhat.com>
Acked-by: James Morris <jmorris@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>