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

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

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

When compiling the module with INVARIANTS we also need INVARIANT_SUPPORT.

Repo-copy of sys/pci/if_en_pci.c to the rest of the midway driver (sys/dev/en)
so that all of the driver sources are in one place. Adjust the configuration
files and the module build.

Convert the midway driver to use busdma. Except for this conversion the
following changes have been done:

- stylify. The original code was too hard to read.
- get rid of a number of compilation options (Adaptec-only, Eni-only, no-DMA).
- more debugging features.
- locking. This is not correct yet in the absence of interface layer locking,
but is correct enough to not to cause lock order reversals.
- remove RAW mode. There are no users of this in the tree and I doubt that
there are any.
- remove NetBSD compatibility code. There was no way to keep NetBSD non-busdma
and FreeBSD busdma code together.
- if_en now buildable as a module.

This has been actively tested on sparc64 and i386 with ENI server and
client cards and an Adaptec card (thanks to kjc).

Reviewed by: mdodd, arr