Add a list_is_singular() predicate

Signed-off-by: Christophe Vu-Brugier <cvubrugier@yahoo.fr>
Add const to isns_tlv_set{,_string}()

Signed-off-by: Christophe Vu-Brugier <cvubrugier@yahoo.fr>
Allow isns_target_register() to register all targets at once

When isns_target_register() is invoked with the special value
'ALL_TARGETS', it registers all iSCSI target with a single iSNS PDU.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@yahoo.fr>
Allow isns_target_deregister() to deregister all targets at once

Also remove isns_deregister().

Signed-off-by: Christophe Vu-Brugier <cvubrugier@yahoo.fr>