ws proxy: also proxy h1 ws to h1 and h2

lws has been able to proxy h2 or h1 inbound connections to an
h1 onward connection for a while now.  It's simple to use just
build with LWS_WITH_HTTP_PROXY and make a mount where the origin
is the onward connection details.  Unix sockets can also be
used as the onward connection.

This patch extends the support to be able to also do the same for
inbound h2 or h1 ws upgrades to an h1 ws onward connection as well.

This allows you to offer completely different services in a
common URL space, including ones that connect back by ws / wss.