NetBSD video(4) support, patch 1 of 3

The video(4) driver provides a Video4Linux2 compatible API for
various video peripherals. This patch propagates HAVE_VIDEOIO if
the sys/videoio.h include file is found, which is the signature of
video(4).
NetBSD video(4) support, patch 2 of 3

* Decouple Video4Linux2 support from Video4Linux as existence of
  v4l2 on a system does not imply support for v4l.
* Don't use V4L's struct video_window in V4L2 code.
* Removed __USE_GNU as comment says:
      /* support for MJPEG is only available with libjpeg and gcc,
         because it's use libjepg and fmemopen()
  so replace with test for fmemopen() if found necessary.
NetBSD video(4) support, patch 3 of 3

xioctl() assumes that ioctl takes int request. Cope with
  int ioctl(int d, unsigned long request, ...)
to avoid "invalid argument".
Don't check for EINTR and replace xioctl with ioctl

This should be safe todo unless we are writing a signal handler.
Fix build warning
Merge pull request #346 from taka-no-me:decouple_V4L2