doesn't really make any difference, except it matches wtmp(5) better.
Don't do anything in pam_sm_close_session(); init(8) will take care of
utmp and wtmp when the tty is released. Clearing them here would make it
possible to create a ghost session by logging in, running 'login -f $USER'
and exiting the subshell.
Sponsored by: DARPA, NAI Labs (but the bugs are all mine)