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

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

Fix build for gcc users by declaring variables for unions in structs which
don't declare a variable. The size before/after this change of the structs
doesn't change with gcc/clang.

Noticed by: several
Suggested by: Gary Jennejohn <gljennjohn@googlemail.com>

- Move videodev headers from compat/linux to contrib/v4l (cp from vendor and
apply diff to compat/linux versions).
- The cp implies an update of videodev2.h to the linux kernel 2.6.34.14 one.

The update makes video in skype v4 work on FreeBSD.

Tested by: Artyom Mirgorodskiy <artyom.mirgorodsky@gmail.com>
(update of header only)

Update videodev2.h to the version from linux kernel 2.6.34.14.
This version is dual-licensed: 3 clause BSD license and GPL v2+.

Add the v4l2 header now that we have an use case for it.