iscsi-target: Avoid OFMarker + IFMarker negotiation

This patch fixes a v4.2+ regression introduced by commit c04a6091


that removed support for obsolete sync-and-steering markers usage
as originally defined in RFC-3720.

The regression would involve attempting to send OFMarker=No +
IFMarker=No keys during opertional negotiation login phase,
including when initiators did not actually propose these keys.

The result for MSFT iSCSI initiators would be random junk in
TCP stream after the last successful login request was been sent
signaling the move to full feature phase (FFP) operation.

To address this bug, go ahead and avoid negotiating these keys
by default unless the initiator explicitly proposes them, but
still respond to them with 'No' if they are proposed.

Reported-by: Dragan Milivojević <galileo@pkm-inc.com>
Bisected-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Tested-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>