ah move more_rx_waiting to wsi scope

Originally this was alright in wsi->u.hdr, because ah implied header
processing.  But since we allowed ah to be held across http
keep-alive transactions if we saw we had more header data, it means
we were trying to read this union member out of scope after it had
transitioned.

Moving the more_rx_waiting member to be a 1-bit bifield in the wsi
solves it and lets us check the state any time later at http
transaction completion.

https://github.com/warmcat/libwebsockets/issues/441



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