This is required to give us access to previously-private fields on
UnknownCmsg.

We stop `clear()`ing the cmsg buffer because nix 0.30.1 transforms it to
a `msghdr` based on the buffer's length, whereas previously nix would do so
based on the buffer's capacity. Additionally, it was only cleared on each
call to `poll_recv_and_rx_time`, rather than before each `rcvmsg` call,
so any fears of leftover data should be alleviated.

This allows users to read `SO_MARK` control message values on incoming
UDP packets. By extension, it attempts to set `SO_RCVMARK` on the
listening socket.

Note that this will only store the first `SO_MARK` value received on a
given connection. We grab the value from the packet which kicks off the
Handshake IOW.