session: set reauth sessions as such

Reauthentication sessions need subtly different behavior than initial
login sessions with regard to pam.  For instance, we want to refresh
existing kerberos credentials instead of establishing new kerberos
credentials.

GDM has all the code in place to do this properly, but lacks the actual
call to mark reauthentication sessions as such.

This commit adds the missing call.

https://bugzilla.gnome.org/show_bug.cgi?id=691269