Add a test for cancelling an active AIO request on a socket.

Add a test for cancelling an active AIO request on a socket.

The older AIO code awakened all pending AIO requests on a socket
when any data arrived.  This could result in AIO daemons blocking on
an empty socket buffer.  These requests could not be cancelled
which led to a deadlock during process exit.  This test reproduces
this case.  The newer AIO code is able to cancel the pending AIO
request correctly.

Reviewed by:	ngie (-ish)
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D4363