sulogin: relabel terminal according to SELinux policy

sulogin: relabel terminal according to SELinux policy

The common SELinux practice is to have a distinct label for terminals in
use by logged in users.  This allows to differentiate access on the
associated terminal (e.g. user_tty_device_t) vs foreign ones (e.g.
tty_device_t or sysadm_tty_device_t).  Therefore the application
performing the user login and setting up the associated terminal should
label that terminal according to the loaded SELinux policy.  Commonly
this is done by pam_selinux(7).  Since sulogin(8) does not use pam(7)
perform the necessary steps manually.

Fixes: https://github.com/util-linux/util-linux/issues/1578
Reviewed-by: James Carter <jwcart2@gmail.com>
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>