forward_route: use close_notify on shutdown

[bneradt]

I notice when running autests on fedora:38, which has a recent version of openssl, that the tls_forward_nonhttp.test.py test fails because openssl s_client complains about a connection being shutdown without close_notify. This issue is explained some in the openssl 3.x docs here:

https://www.openssl.org/docs/man3.0/man1/openssl-s_client.html

-ignore_unexpected_eof Some TLS implementations do not send the mandatory close_notify alert on shutdown. If the application tries to wait for the close_notify alert but the peer closes the connection without sending it, an error is generated. When this option is enabled the peer does not need to send the close_notify alert and a closed connection will be treated as if the close_notify alert was received. For more information on shutting down a connection, see SSL_shutdown(3).

In case it's helpful, here's s_client's output concerning this:

<other output from test>
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This is a reply                                                      
20447898FFFF0000:error:0A000126:SSL routines:ssl3_read_n:unexpected eof while reading:ssl/record/rec_layer_s3.c:303:

I'm currently patching the test to add -ignore_unexpected_eof so the test will run on these newer systems, but I'm filing this ticket because we should investigate this issue and see whether ATS needs to be updated to properly notify the closing of connections.

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