target: Add se_portal_group->tpg_auth_group

This patch adds an optional /auth/ configfs group to TPG context that
can be used by fabrics like iscsi-target for TPG demo-mode
authentication.

Cc: Dax Kelson <dkelson@gurulabs.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>