kernel - Rewrite the callout_*() API

* Rewrite the entire API from scratch and improve compatibility
with FreeBSD. This is not an attempt to achieve full API compatibility,
as FreeBSD's API has

kernel - Rewrite the callout_*() API

* Rewrite the entire API from scratch and improve compatibility
with FreeBSD. This is not an attempt to achieve full API compatibility,
as FreeBSD's API has unnecessary complexity that coders would frequently
make mistakes interpreting.

* Remove the IPI mechanisms in favor of fine-grained spin-locks instead.

* Add some robustness features in an attempt to track down corrupted
callwheel lists due to originating subsystems freeing structures out
from under an active callout.

* The code supports a full-blown type-stable/adhoc-reuse structural
separation between the front-end and the back-end, but this feature
is currently not operational and may be removed at some future point.
Instead we currently just embed the struct _callout inside the
struct callout.

* Replace callout_stop_sync() with callout_cancel().

* callout_drain() is now implemented as a synchronous cancel instead
of an asynchronous stop, which is closer to the FreeBSD API and
expected operation for ported code (usb stack in particular). We
will just have to fix any deadlocks which we come across.

* Retain our callout_terminate() function as the 'better' way to
stop using a callout, as it will not only cancel the callout but
also de-flag the structure so it can no longer be used.

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kernel: Remove unused *.h files from SRCS in kernel module Makefiles.

They were found by checking the preprocessed code of the files
in SRCS to see if the header was included at some point.

After r

kernel: Remove unused *.h files from SRCS in kernel module Makefiles.

They were found by checking the preprocessed code of the files
in SRCS to see if the header was included at some point.

After removal, the preprocessed source of a build with the old
Makefiles was compared against one with the changes (for various
kernel configurations and when building just from /usr/src) to
verify that the commit leads to no functional change.

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kernel: Use NULL for pointers in DRIVER_MODULE().

kernel/virtio: Remove some last traces of virtio_if.m.

It was removed in b817dce2e30a257b86b253a698e63a418758d76d.

kernel: Bring in a port of FreeBSD's virtio_random(4) driver.

Tested-by: Diederik de Groot <info@talon.nl>