MFC r344636: Refactor command ordering/blocking mechanism in CTL.

Replace long per-LUN queue of blocked commands, scanned on each command
completion and sometimes even twice, causing up to O(n^^2) processing cost,
by much shorter per-command blocked queues, scanned only when respective
command completes, and check only commands before the previous blocker,
reducing cost to O(n).

While there, unblock aborted commands to make them "complete" ASAP to be
removed from the OOA queue and so not waste time ordering other commands
against them. Aborted commands that were not sent to execution yet should
have no visible side effects, so this is safe and easy optimization now,
comparing to commands already in processing, which are a still pain.

Together those two optimizations should fix quite pathological case, when
due to backend slowness CTL accumulated many thousands of blocked requests,
partially aborted by initiator and so supposedly not even existing, but
still wasting CTL CPU time.

MFC r312603: Add initial support for CTL module unloading.

It is only a first step and not perfect, but better then nothing.
The main blocker is CAM target frontend, that can not be unloaded,
since CAM does not have mechanism to unregister periph driver now.

MFC r312291, r312669:
Make CTL frontends report kern_data_resid for under-/overruns.

It seems like kern_data_resid was never really implemented. This change
finally does it. Now frontends update this field while transferring data,
while CTL/backends getting it can more flexibly handle the result.
At this point behavior should not change significantly, still reporting
errors on write overrun, but that may be changed later, if we decide so.

CAM target frontend still does not properly handle overruns due to CAM API
limitations. We may need to add some fields to struct ccb_accept_tio to
pass information about initiator requested transfer size(s).

MFC r312232: Add under-/overrun support to IOCTL and CAM SIM frontends.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

sys/cam: spelling fixes in comments.

No functional change.

Move ioctl frontend defines where they belong.

Remove some dead code found by Clang analyzer.

Remove couple excess SGLIST I/O flags.

Those flags duplicated respective (sg_entries > 0) values.

Frontends don't need to set errors themselves.

Reimplement CTL High Availability.

CTL HA functionality was originally implemented by Copan many years ago,
but large part of the sources was never published. This change includes
clean room implementation of the missing code and fixes for many bugs.

This code supports dual-node HA with ALUA in four modes:
- Active/Unavailable without interlink between nodes;
- Active/Standby with second node handling only basic LUN discovery and
reservation, synchronizing with the first node through the interlink;
- Active/Active with both nodes processing commands and accessing the
backing storage, synchronizing with the first node through the interlink;
- Active/Active with second node working as proxy, transfering all
commands to the first node for execution through the interlink.

Unlike original Copan's implementation, depending on specific hardware,
this code uses simple custom TCP-based protocol for interlink. It has
no authentication, so it should never be enabled on public interfaces.

The code may still need some polishing, but generally it is functional.

Relnotes: yes
Sponsored by: iXsystems, Inc.

Make most of port methods optional and remove bunch of dummies.

Move "ioctl" CAM frontend into separate file.

It has nothing to share with too huge ctl.c other then device descriptor,
but even that may be counted as design error that may be fixed later.
At some point we may even want to have several ioctl ports.