v4l2: rutime libv4l2 support

If compiled with libv4l2 support, first try using the kernel v4l2 api
and then fallback to libv4l2 if it didn't work. You can force using
libv4l2 with --v4l2-use-libv4l2. (libv4l2 broke the v4l2 access for
some people. This should be backported to 0.9.9)