V4L/DVB (5146): Make VIDIOC_INT_[SG]_REGISTER ioctls no longer internal only

The direct register access ioctls were defined as kernel internal only,
but they are very useful for debugging hardware from userspace and are
used as such.  Officially export them.

VIDIOC_INT_[SG]_REGISTER is renamed to VIDIOC_DBG_[SG]_REGISTER
Definition of ioctl and struct v4l2_register is moved from v4l2-common.h
to videodev2.h.

Types used in struct v4l2_register are changed to the userspace
exportable versions (u32 -> __u32, etc).

Use of VIDIOC_DBG_S_REGISTER requires CAP_SYS_ADMIN permission, so move
the check into the video_ioctl2() dispatcher so it doesn't need to be
duplicated in each driver's call-back function. CAP_SYS_ADMIN check is
added to pvrusb2 (which doesn't use video_ioctl2).

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>