libwebsockets

ws: wsi unusability only applies when no pending

We basically hear about HUP out-of-band with respect to
pending rx... now we mark the wsi as "unusable" once we
see the HUP - the connection has ended.

This means we have to differentiate between the wsi
being unusable for new things now, like replying, and
what it has already sent still being servicible and
pending.