Add an additional 'initiator' option to shareiscsi
that is only available for LIO.

One possible problem with this commit: It will make the compatibility
between the different iSCSI implementations questionable - can't
switch between them easily (it will ONLY be availible in ZoL/LIO).  I
COULD make the option be silently ignored (instead of forcibly
rejected if used and not LIO).

But that still doesn't solve the (possible) problem between ZoL and
OpenZFS (setting the option, will possibly introduce problems when
importing the pool on something else than ZoL).

So some more discussion might be needed..
Misc iSCSI fixes/cleanups/attempts to get 'zfs rename' to work.

!! Doesn't - a 'zfs rename' works, but most often than not, the rename
!! IOCTL hangs. Not every time and not always on the first dataset being
!! renamed... Seems a little to random to me.