playground: Signal 'eof' instead of closing the channel with fswrite.
ws: Hang onto query string in CockpitWebResponse

So that we can use it in various handlers

Closes #1539
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
base: Fix use of Uint8Array with out the new operator

Get this error message:

  Uncaught TypeError: Constructor Uint8Array requires 'new'

Closes #1542
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
common: Allow parsing command messages without a command

If no command parameter is specified to cockpit_transport_parse_command()
allow skipping the command field.
ws: Support for sideband channel WebSockets

These are for channels that need a lot of throughput like SPICE
and VNC, where we want to avoid splicing and prefixing everything
with a channel id.

These are separate WebSockets that are individual channels. They
are opened with a query string containing the JSON encoded "open"
command message. Once open, all data sent through the WebSocket
is part of the payload of that channel.