MINOR: ssl-sock: pass the CO_SFL_MSG_MORE info down the stack

MINOR: ssl-sock: pass the CO_SFL_MSG_MORE info down the stack

Despite having replaced the SSL BIOs to use our own raw_sock layer, we
still didn't exploit the CO_SFL_MSG_MORE flag which is pretty useful to
avoid sending incomplete packets. It's particularly important for SSL
since the extra overhead almost guarantees that each send() will be
followed by an incomplete (and often odd-sided) segment.

We already have an xprt_st set of flags to pass info to the various
layers, so let's just add a new one, SSL_SOCK_SEND_MORE, that is set
or cleared during ssl_sock_from_buf() to transfer the knowledge of
CO_SFL_MSG_MORE. This way we can recover this information and pass
it to raw_sock.

This alone is sufficient to increase by ~5-10% the H2 bandwidth over
SSL when multiple streams are used in parallel.