libwebsockets

POST handling dont autocomplete transaction

Until now lws has finished the HTTP transaction when the POST body was
completely received.

However that needlessly makes it impossible to send a HTTP 200 and a
response without a redirect.

This changes lws behaviour after sending the LWS_CALLBACK_HTTP_BODY_COMPLETION
callback to no longer terminate the HTTP transaction.

If you want the old behaviour, you can terminate the transaction with
lws_http_transaction_completed() in the LWS_CALLBACK_HTTP_BODY_COMPLETION
callback.

Otherwise it's now possible to call lws_callback_on_writable() from
LWS_CALLBACK_HTTP_BODY_COMPLETION.

Signed-off-by: Andy Green <andy@warmcat.com>