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.

Sponsored by:	The FreeBSD Foundation
Document the new 'vm-image' target, and associated release.conf
variables.

Sponsored by:	The FreeBSD Foundation
Remove two stray comments added during the initial iterations
of testing, no longer needed.

Sponsored by:	The FreeBSD Foundation
Reintegrate head@r272414

Sponsored by:	The FreeBSD Foundation
Remove the first argument to panic(), which was initially
intended to be the exit code, however when a non-zero exit
code was returned to release/Makefile, this would prevent
any remaining (and possibly successful) stages from being
attempted.

Sponsored by:	The FreeBSD Foundation
If the vm-base target fails, prevent the vm-image target
from being run since it cannot possibly succeed.

Sponsored by:	The FreeBSD Foundation
Document new table values.

Sponsored by:	Yandex LLC
Make linear_skipto turned off by default.
Bump max rule size to 512 opcodes.
Switch ipfw to use rmlock for runtime locking.
Add "ipfw_ctl3" FEATURE to indicate presence of new ipfw interface.
Sync to HEAD@r272516.
Please GCC by specifying proper cast.
Fix build with gcc.
Fix GCC wardnings.
IFC @r272481
Fix tracked interface list retrieval.
Sync to HEAD@r272609.
* Fix crash in interface tracker due to using old "linked" field.
* Ensure we're flushing entries without any locks held.
* Free memory in (rare) case when interface tracker fails to register ifp.
* Add KASSERT on table values refcounts.
Notify table algo aboute runtime data change on table flush.