cap_util: flesh out capture main() to capture some frames using epoll().

it should be a better test/example now.
fix (apparently) invalid usage of null_buffers() with async_read(); use socket->async_read_some() instead.
cap_util: track (and print in main()) read_but_dropped_frames.

should be zero, but appears to be non-zero due to V4L2/epoll() kernel bug: EPOLLIN only triggered when *all* buffers are full for ucv driver. google: V4L2 epoll bug. may be fixed in kernels more recent than 3.2?
cap_app: back to using polling for video input to work around V4L2/epoll() kernel bug.
fix various help typos and one type_id typo