socket: introduce MNL_RING_MSGHDR macro
socket: introduce mnl_socket_poll_rx
nlmsg: introduce mnl_nlmsg_batch_reset_buffer for mmap
examples: update for mnl_socket_set_ring change
examples: update using MNL_RING_MSGHDR
examples: update for mnl_socket_poll_rx
examples: separate mmap
examples: adjust nl_mmap_req
examples: add for mmap exclude rtnl