Fix multiple problems around LUN disable under load.

• Move private data about ATIOs/INOTs from per-LUN to per-channel data.

This allows active commands to continue operation after LUN destruction.
This also simplifies lookup of the data by tag in some situations.

• Unify three restart_queue processing implementations.
• Complete all ATIOs from restart_queue on LUN disable.
• Delete ATIO private data when command completed or aborted, not depending

on the ATIO being requeued, that was ugly hack and could never happen. CAM
should always call ether XPT_CONT_TARGET_IO with status or XPT_ABORT.

• Implement XPT_ABORT for queued ATIOs/INOTs to allow CAM do graceful

shutdown, not depending on LUN disable, as it is done in ahd(4)/targ(4).

• Unify isp_endcmd() arguments to make it more usable in generic code.
• Remove never really used LUN state reference counter.

MFC after: 2 weeks