Linux LIO/targetcli support

Linux LIO/targetcli support

Introducing LIO/targetcli support allowing to use recent linux
distributions as iSCSI targets for ZFS volumes.

In order for this to work, two preconditions have to be met:

1. the portal has to be set up correctly using targetcli
2. the initiator has to be authorized to connect to the target
   based on the initiator's InitiatorName

When adding a LIO iSCSI target, a new "LIO target portal group" field needs
to be correctly populated in the "Add: ZFS over iSCSI" popup, containing the
fitting "LIO target portal group" name (typically something like 'tpg1').

Signed-Off-By: Udo Rader <udo.rader@bestsolution.at>
Tested-by: Stoiko Ivanov <s.ivanov@proxmox.com>