MFC r336634: MFV CK@r336629: Import CK as of commit 1c1f9901c2dea7a883342cd03d3906a1bc482583

This adds CK_SLIST_INSERT_PREVPTR and CK_SLIST_REMOVE_PREVPTR macros
as well as ck_pr_dec_is_zero family of functions.

MFC r334189: Import CK as of commit 0f017230ccc86929f56bf44ef2dca93d7df8076b

This brings us the renaming of fields in ck_queue, so that our own
LIST/SLIST/TAILQ/etc won't accidentally work with them.

MFC r329388, r331441 and r331898, to bring the -CURRENT ck version.
r329388:
Define CK_MD_TSO for the relevant arches (i386, amd64 and sparc64).
Defaulting to CK_MD_RMO has the unfortunate side effect of generating
memory barriers that are useless on those arches, and the even more
unfortunate side effect of generating lfence/sfence/mfence on i386, even
if older CPUs don't support it.
This should fix the panic reported when using IPFW on a Pentium 3.
Note that mfence and sfence might still be used in a few case, but that
shouldn't happen in FreeBSD right now, and should be fixed upstream first.

r331441:
In __sync_bool_compare_and_swap(), return true if the returned value is the
same as the expected one, not the desired one.

r331898:
Import CK as of commit b19ed4c6a56ec93215ab567ba18ba61bf1cfbac8
It should fix ck_pr_[load|store]_ptr on mips and riscv, make sure no
*fence instructions are used on i386, as older cpus don't support it, and
make sure we don't rely on gcc builtins that can lead to calls to
libatomic when linked with -O0.

MFC revisions r309268, r309260, r309264, r309266, r309267, r309270, r310846,
r314435, r314564, r316665, r316691, r316702.
Those import ConcurrencyKit in the FreeBSD kernel.
A few people have showed interest in this being MFC'd, so here we go.

Import Concurrency Kit in the kernel.
CK is a toolkit providing different lockfree algorithms/data structures.
More information can be found here : www.concurrencykit.org

Import concurrencykit as of commit 566bb28dba963a1904e0889b74fe7005a5bc5eb8