aio: Interlock with listen(2)

[markj]

Summary

soo_aio_queue() did not handle the possibility that the provided socket
is a listening socket. Up until recently, to fix this one would have to
acquire the socket lock first and check, since the socket buffer locks
were destroyed by listen(2).

Now that the socket buffer locks belong to the socket, simply check
SOLISTENING(so) after acquiring them, and make listen(2) return an error
if any AIO jobs are enqueued on the socket.

Add a couple of simple regression test cases.

Note that this fixes things only for the default AIO implementation;
cxgbe has a separate pru_aio_queue implementation which needs to be
fixed.

Reported by: [e-mail]
Reported by: [e-mail]