su: use full tty path for PAM_TTY

su: use full tty path for PAM_TTY

pam_set_item() man page:
 PAM_TTY
   The terminal name: prefixed by /dev/ if it is a device file;
   for graphical, X-based, applications the value for this item
   should be the $DISPLAY variable.

It seems for example pam_timestamp module is not robust enough to
differentiate between /dev/ and pty/0 and it assumes that '/' in the
path always means '/dev/' prefix ...

Fixes: https://github.com/karelzak/util-linux/issues/1242
Signed-off-by: Karel Zak <kzak@redhat.com>