Internalise handling of virtualised hook points inside
hhook_{add|remove}_hook_lookup() so that khelp (and other potential API
consumers) do not have to care when they attempt to (un)hook a particular hook
point identified by id and type.

Reviewed by:	scottl
MFC after:	1 week
Properly track the different reasons new I/O is temporarily disabled, and
only re-enable I/O when all reasons have cleared.

sys/dev/xen/blkfront/block.h:
	In the block front driver softc, replace the boolean
	XBDF_FROZEN flag with a count of commands and driver global
	issues that freeze the I/O queue.  So long xbd_qfrozen_cnt
	is non-zero, I/O is halted.

	Add flags to xbd_flags for tracking grant table entry and
	free command resource shortages.  Each of these classes can
	increment xbd_qfrozen_cnt at most once.

	Add a command flag (XBDCF_ASYNC_MAPPING) that is set whenever
	the initial mapping attempt of a command fails with EINPROGRESS.

sys/dev/xen/blkfront/blkfront.c:
	In xbd_queue_cb(), use new XBDCF_ASYNC_MAPPING flag to definitively
	know if an async bus dmamap load has occurred.

	Add xbd_freeze() and xbd_thaw() helper methods for managing
	xbd_qfrozen_cnt and use them to implement all queue freezing logic.

	Add missing "thaw" to restart I/O processing once grant references
	become available.

Sponsored by:	Spectra Logic Corporation
Add a private KPI between hhook and khelp that allows khelp modules to insert
hook functions into hhook points which register after the modules were loaded -
potentially useful during boot or if hhook points are dynamically registered.

MFC after:	1 week
Cleanup and simplification in khelp_{register|deregister}_helper(). No
functional changes.

MFC after:	1 week
Make support for atomics on ARM complete.

Provide both __sync_*-style and __atomic_*-style functions that perform
the atomic operations on ARMv5 by using Restartable Atomic Sequences.

While there, clean up some pieces of code where it's sufficient to use
regular uint32_t to store register contents and don't need full reg_t's.
Also sync this back to the MIPS code.
Stick to using the documented atomic(9) API.

The atomic_store_ptr() function is not part of the atomic(9) API. We
only provide a version with a release barrier.
Remove conflicting macros from SPARC64's atomic(9) header.

The atomic_load() and atomic_store() macros conflict with the equally
named macros from <stdatomic.h>. Remove them, as they are only used to
implement functions that are not present on any of the other
architectures.
Let ARM use the custom tailored atomic intrinsics.
Pull in r184040 from upstream clang trunk:

  Emit native implementations of atomic operations on FreeBSD/armv6.

  Just like on Linux, FreeBSD/armv6 assumes the system supports
  ldrex/strex unconditionally. It is also used by the kernel. We can
  therefore enable support for it, like we do on Linux.

  While there, change one of the unit tests to explicitly test against
  armv5 instead of armv7, as it actually tests whether libcalls are
  emitted.
Move hhook's per-vnet initialisation to an earlier SYSINIT SI_SUB stage to
ensure all per-vnet related hhook initialisation is completed prior to any
virtualised hhook points attempting registration.

vnet_register_sysinit() requires that a stage later than SI_SUB_VNET be chosen.
There are no per-vnet initialisors in the source tree at this time which run
earlier than SI_SUB_INIT_IF. A quick audit of non-virtualised SYSINITs indicates
there are no subsystems pre SI_SUB_MBUF that would likely be interested in
registering a virtualised hhook point.

Settle on SI_SUB_MBUF as hhook's per-vnet initialisation stage as it's the first
overtly network-related initilisation stage to run after SI_SUB_VNET. If a
subsystem that initialises earlier than SI_SUB_MBUF ends up wanting to register
virtualised hhook points in future, hhook's use of SI_SUB_MBUF will need to be
revisited and would probably warrant creating a dedicated SI_SUB_HHOOK which
runs immediately after SI_SUB_VNET.

MFC after:	1 week
Now that the necessary infrastructure is in place to ensure hhook points which
register after a khelp module will get hooked, move khelp module initialisation
to the earlier SI_SUB_KLD stage.

MFC after:	1 week
Pull in r183926 from LLVM trunk:

  Allow clang to build __clear_cache on ARM.

  __clear_cache is special. It needs no signature, but is a real function in
  compiler_rt or libgcc.

  Patch by Andrew Turner.

This allows us to build the __clear_cache function in compiler-rt.
Build __clear_cache on ARM with clang now it supports it.
Restore use of polling mode for disk cache flush in case of kernel panic.
While I am not sure that any extra hardware access is a good idea after
panic, that is an existing behaviour that should better work correctly.
Style/comments.
Remove CVS from the base system.

Discussed with:	many
Reviewed by:	peter, zi
Approved by:	core
Regenerate after CVS removal (and it seems other missed changes)
Let hastd use C11 atomics.

C11 atomics now work on all the architectures. Have at least a single
piece of software in our base system that uses C11 atomics. This
somewhat makes it less likely that we break it because of LLVM imports,
etc.
sh: Add tests for 'local -' (save shell options).
Return ENETDOWN when the parent interface is down.

MFC after:	1 week