enable websocket upgrades without the html5 client

[totaam]

Currently the server will not "upgrade" sockets unless the html5 client is installed (the _html flag):

xpra/xpra/server/server_core.py

Line 1549 in aa1e864

http_upgrade = socket_options.get(http_protocol, self._html) not in FALSE_OPTIONS

Technically, there is no reason for this:

• one could host the html5 client somewhere else
• one could want to use the python client in websocket mode to go through a proxy
  etc

The current workaround is to add http=true (or https=true for ssl) to the socket options, ie:
--bind-tcp=0.0.0.0:10000,http=true

[totaam]

This is hard because in a lot of cases, we process socket upgrades to SSL without knowing if the socket is going to be:

• a websocket connection
• an https client
• a plain ssl connection

And each one of these can have its own toggle.

But it is needed because we currently assume too much. For example, we publish mdns records based on generic socket type mappings rather than what we end up using for deciding whether to upgrade a socket or not.

[totaam]

Mostly done and seems to work, but this needs more testing and therefore was not added to the new v5.x branch.