deprecate LWS_SEND_BUFFER_POST_PADDING

The only guy who cared about this for a long while
(since I eliminated the pre-standard protocol variants)
was sending a close frame.

 - Set it to 0 so old code remains happy.  It only affects
user code buffer commit, if there's overcommit no harm
done so no effect directly on user ABI.

 - Remove all uses inside the library.  The sample apps
don't have it any more and that's the recommendation now.

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