[SCSI] libiscsi: make can_queue configurable

This patch allows us to set can_queue and cmds_per_lun from userspace
when we create the session/host. From there we can set it on a per
target basis. The patch fully converts iscsi_tcp, but only hooks
up ib_iser for cmd_per_lun since it currently has a lots of preallocations
based on can_queue.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: Roland Dreier <rdreier@cisco.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>