coap-server: GnuTLS does not correctly detect closure by a client using TCP
coap-server: GnuTLS does not correctly detect closure by a client using TCP

To reproduce:-

Run a GnuTLS-based coap-server with option `-n` and connect with
`openssl s_client` to port 5684. The handshake works and see binary data
(the CoAP/TCP CSM signaling). Now, then interrupt the openssl s_client process
with SIGINT, This generates a lot of these messages:

Feb 14 19:22:39 DEBG ASSERT: ../../lib/record.c[_gnutls_recv_int]:1766
Feb 14 19:22:39 DEBG ASSERT: ../../lib/buffers.c[_gnutls_io_read_buffered]:589
Feb 14 19:22:39 DEBG ASSERT: ../../lib/record.c[_gnutls_recv_int]:1766
Feb 14 19:22:39 DEBG ASSERT: ../../lib/buffers.c[_gnutls_io_read_buffered]:589
Feb 14 19:22:39 DEBG ASSERT: ../../lib/record.c[_gnutls_recv_int]:1766
Feb 14 19:22:39 DEBG ASSERT: ../../lib/buffers.c[_gnutls_io_read_buffered]:589
Feb 14 19:22:39 DEBG ASSERT: ../../lib/record.c[_gnutls_recv_int]:1766
Feb 14 19:22:39 DEBG ASSERT: ../../lib/buffers.c[_gnutls_io_read_buffered]:589

An OpenSSL-based coap-server gets a clean disconnect and closes the session as
expected.

src/coap_gnutls.c:

Return correct response when coap_sock_read() is called by GnuTLS internal
Pull and the TCP session is closed so that the coap session is closed
correctly

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap