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

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

Fix make buildworld -DMODULES_WITH_WORLD on i386

INET doesn't belong to opt_inet6.h: INET6 belongs to it.

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

- add support for IPX (tested with mount -t nwfs and mars_nwe),
IP fast forwarding, SIOCGIFADDR, setting hardware address (not currently
enabled in cm driver), multicasts (experimental)
- add ARC_MAX_DATA, use IF_HANDOFF, remove arc_sprintf() and some unused
variables
- if_simloop logic is made more similar to ethernet
- drop not ours packets early (if we are not in promiscous mode)

Submitted by: mark tinguely (partially)

Export symbols that constitute APIs defined by these
modules.

Note that in the case of 'mii' the API is not clearly
defined, and the symbols exported represent a subset
defined by current usage.

- generic Arcnet framework
- device driver for SMC COM90cx6 Arcnet network adapters

Obtained from: NetBSD