MFC r271588:  Update CAM CCB accounting for the new status quo.

devq_openings counter lost its meaning after allocation queues has gone.
held counter is still meaningful, but problematic to update due to separate
locking of CCB allocation and queuing.

To fix that replace devq_openings counter with allocated counter.  held is
now calculated on request as difference between number of allocated, queued
and active CCBs.
MFC r272414:
  Merge the following revisions from ^/projects/release-vmimage:

  r272234, r272236, r272262, r272264, r272269, r272271, r272272,
  r272277, r272279, r272376, r272380, r272381, r272392, r272234,
  r272412:

  r272234:
    Initial commit to include virtual machine images as part
    of the FreeBSD release builds.

    This adds a make(1) environment variable requirement,
    WITH_VMIMAGES, which triggers the virtual machine image
    targets when not defined to an empty value.

    Relevant user-driven variables include:

     o VMFORMATS:	The virtual machine image formats to create.
       Valid formats are provided by running 'mkimg --formats'

     o VMSIZE:	The size of the resulting virtual machine
       image.  Typical compression is roughly 140Mb, regardless
       of the target size (10GB, 15GB, 20GB, 40GB sizes have been
       tested with the same result).

     o VMBASE:	The prefix of the virtual machine disk images.
       The VMBASE make(1) environment variable is suffixed with
       each format in VMFORMATS for each individual disk image, as
       well as '.img' for the source UFS filesystem passed to
       mkimg(1).

    This also includes a new script, mk-vmimage.sh, based on how
    the VM images for 10.0-RELEASE, 9.3-RELEASE, and 10.1-RELEASE
    were created (mk-vmimage.sh in ^/user/gjb/thermite/).

    With the order in which the stages need to occur, as well as
    sanity-checking error cases, it makes much more sense to
    execute a shell script called from make(1), using env(1) to
    set specific parameters for the target image than it does to
    do this in make(1) directly.

  r272236:
    Use VMBASE in place of a hard-coded filename in the CLEANFILES
    list.

  r272262:
    Remove a 'set -x' that snuck in during testing.

  r272264:
    release/Makefile:
      Connect the virtual machine image build to the release
      target if WITH_VMIMAGES is set to a non-empty value.

    release/release.sh:
      Add WITH_VMIMAGES to RELEASE_RMAKEFLAGS.

    release/release.conf.sample:
      Add commented entries for tuning the release build if the
      WITH_VMIMAGES make(1) environment variable is set to
      a non-empty value.

  r272269:
    release/Makefile:
      Include .OBJDIR in DESTDIR in the vm-base target.

    release/release.sh:
      Provide the full path to mddev.

  r272271:
    Fix UFS label for the root filesystem.

  r272272:
    Remove comments left in accidentally while testing, so the
    VM /etc/fstab is actually created.

  r272277:
    Remove the UFS label from the root filesystem since it is added
    by mkimg(1) as a gpt label, consistent with the fstab(5) entry.

  r272279:
    Comment cleanup in panic() message when mkimg(1) does not support
    the requested disk image format.

  r272376:
    Separate release/scripts/mk-vmimage.sh to machine-specific
    scripts, making it possible to mimic the functionality for
    non-x86 targets.

    Move echo output if MAKEFLAGS is empty outside of usage().

    Remove TARGET/TARGET_ARCH evaluation.

  r272380:
    Avoid using env(1) to set values passed to mk-vmimage.sh,
    and instead pass the values as arguments to the script,
    making it easier to run this by hand, without 'make release'.

    Add usage_vm_base() and usage_vm_image() usage helpers.

  r272381:
    After evaluating WITH_VMIMAGES is non-empty, ensure
    the mk-vmimage.sh script exists before running it.

  r272392:
    Add WITH_COMPRESSED_VMIMAGES variable, which when set enables
    xz(1) compression of the virtual machine images.

    This is intentionally separate to allow more fine-grained
    tuning over which images are compressed, especially in cases
    where compressing 20GB sparse images can take hours.

  r272412:
    Document the new 'vm-image' target, and associated release.conf
    variables.

  r272413:
    Remove two stray comments added during the initial iterations
    of testing, no longer needed.

Sponsored by:	The FreeBSD Foundation
Update the FreeBSD versions in the stable/10 errata documentation
to reflect reality.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Clean up the stable/10 errata document now that releng/10.1
has precedence over releng/10.0.

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