inets: Teach httpc to honour `Connection: close` from server

From https://tools.ietf.org/html/rfc7230#section-6.6

> A client that receives a "close" connection option MUST cease
  sending requests on that connection and close the connection after
  reading the response message containing the "close"; if additional
  pipelined requests had been sent on the connection, the client
  SHOULD NOT assume that they will be processed by the server.

Notes on tests:

* The new tests are added only in group sim_http and not sim_https
  because the same test approach appears to be not valid because when
  processing the first response the server already sent data (> 0) for
  the TLS/SSL handshake;

* The order of tests is relevant as it appears some test cases leave
  reusable sessions behind.

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