MFC r297685: Add option to specify built-in keymap for kbdmux

PR: 153459
Submitted by: swell.k@gmail.com

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Fix our ioctl(2) implementation when the argument is "int". New
ioctls passing integer arguments should use the _IOWINT() macro.
This fixes a lot of ioctl's not working on sparc64, most notable
being keyboard/syscons ioctls.

Full ABI compatibility is provided, with the bonus of fixing the
handling of old ioctls on sparc64.

Reviewed by: bde (with contributions)
Tested by: emax, marius
MFC after: 1 week

Provide ability to disable kbdmux(4) with the hint.kbdmux.0.disabled="1" hint.
Document hint in the kbdmux(4) man page.

Requested by: scottl
MFC after: 1 day

Let modules use the kernel's opt_*.h files if built along with
the kernel by wrapping all targets for fake opt_*.h files in
.if defined(KERNBUILDDIR). Thus, such fake files won't be
created at all if modules are built with the kernel.

Some modules undergo cleanup like removing unused or unneeded
options or .h files, without which they wouldn't build this way
or the other.

Reviewed by: ru
Tested by: no binary changes in modules built alone
Tested on: i386 sparc64 amd64

kbdmux(4) keyboard multiplexer integration

o Add sys/dev/kbdmux/kbdmux.c to the source tree

o Add sys/modules/kbdmux/Makefile to the source tree

These are not yet connected to the build. Man page and other changes to follow.

MFC after: 1 week