Medium: RA: iSCSITarget: reintroduce "tid" resource parameter

Much as I liked the idea of referring to targets just by target IQN,
using "dynamic" target IDs unfortunately breaks failover for tgt, at
least with some iSCSI initiators. The details are explained here:

http://lists.wpkg.org/pipermail/stgt/2009-July/003067.html

Consequently, I'm reintroducing the tid parameter for iSCSITarget. It
is required only on tgt, and optional on IET.

iSCSILogicalUnit is unchanged. It still figures out the correct tid
from the configured target_iqn.