close add api to control sent close frame contents

This adds an api lws_close_reason() which lets you control what will
be sent in the close frame when the connection is closed by returning
nonzero from the user callback.

The test server demo is extended to prove it works in both directions.

With this, we should have nice close support.

https://github.com/warmcat/libwebsockets/issues/196



Signed-off-by: Andy Green <andy.green@linaro.org>