video: add generic video(4) capture framework

Add a new video(4) framework that provides /dev/videoN, buffer
management, mmap lifetime, and V4L2 ioctl dispatch for video capture
drivers. Hardware drivers implement struct video_hw_ops callbacks
and use video_buf_acquire/write/done to deliver frames.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D58367