iscsi-target: Add tpg_enabled_sendtargets for disabled discovery

This patch adds a new tpg_enabled_sendtargets configfs attribute
to allow in-band sendtargets discovery information to include
target-portal-groups (TPGs) in !TPG_STATE_ACTIVE state.

This functionality is useful for clustered iSCSI targets, where
TPGTs handled on remote cluster nodes should be advertised in
the SendTargets response.

By default, this new attribute retains the default behaviour of
existing code which to ignore portal-groups in !TPG_STATE_ACTIVE
state.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>