tests: client/server rewrites

For `server.c` the changes are fairly minor since it was already
a relatively straight-forward and self-contained example:

* Handle a potential `EAGAIN` `demo_result` from `write_tls()`.
* Add a `server.h` that is presently unused, but allows keeping the
  compilation rules simple by treating server/client symmetrically.
* Break the connection handling loop when we've both sent a response
  and the rustls connection requires no more writes. This effectively
  closes the connection after a response has been written, without
  waiting on the peer to do so. We want to do this since we don't
  process the HTTP request to learn if the client wanted `Connection:
  keep-alive` or `Connection: close`.

For `client.c`, the changes are more extensive:

* Add a `client.h` so we can forward declare everything interesting.
  This allows `client.c` to match our preferred Rust standard of "top
  down ordering"
* Extract out a `demo_client_options` struct and a `options_from_env()`
  function for handling options based on the environment.
* Extract out a `new_tls_config()` function that takes a pointer to
  `demo_client_options` and returns a `rustls_client_config`.
* Extract out a `demo_client_request_options` struct for per-request
  options (hostname, port, path, whether to use vectored I/O).
* Pull out a `demo_client_connection` struct for managing the state
  associated with a connection (socket fd, rustls_connection, conndata,
  closing stae, etc).
* Rework existing logic around the new types.
* Simplify the request handling to better match tls-client-mio.rs in the
  Rustls examples. Notably we _do not_ process the HTTP response,
  instead we just read whatever data we get and blast it to stdout.
  A new timeout on `select()` ensures that if the server doesn't close
  the connection after writing a response we will time out waiting for
  more data and do it ourselves. With the update to server.c to close
  the connection after writing a response this won't kick in, but is
  helpful for testing against servers that may let the conn linger even
  though we send `Connection: close`.
* Care is taken to still treat unclean closure as an error condition.
* Various other small improvements are made where possible.