Teach ctl_add_initiator() to dynamically allocate IIDs from pool.

If port passed negative IID value, the function will try to allocate IID
from the pool of unused, based on passed wwpn or name arguments.  It does
all its best to make IID unique and persistent across reconnects.

This makes persistent reservation properly work for iSCSI.  Previously,
in case of reconnects, reservation could be unexpectedly lost, or even
migrate between intiators.