allow the proxy to start new sessions

[Antoine Martin]

Following the improvements to the proxy server in r13793, r13795, r13574, r13542, #1264, SSL (#1252), etc.. make it possible to start new server sessions via a proxy server.

[Antoine Martin]

Done in r13798.

Example:

xpra proxy :10 --tcp-auth=allow --bind-tcp=0.0.0.0:10000

On the client:

xpra start tcp/username:password@127.0.0.1:10000/ --start-child=xterm --exit-with-children

Will ask the proxy to start a new session, which you can see with "xpra list".
It will survive the proxy server if the proxy is killed.

Things we could do to take this further:

• pass more options to the new server instance (easy), maybe even including "bind-tcp" options?
• identify sessions by name somehow?
• #408 / start-on-demand option which would re-use a session if one is found

etc..

@afarr: just a FYI.

[Antoine Martin]

r13892 adds support for specifying the display at the end of the URL, and:

• support for start-desktop, ie:

  xpra start-desktop tcp/username:password@127.0.0.1:10000/200 --start-child=fluxbox --exit-with-children
  

Will start a fluxbox full desktop on display ":200".

• support for shadow, ie:

  xpra shadow tcp/username:password@127.0.0.1:10000/1
  

Now that the user can specify the display to shadow, this works on multi-user systems where there may be more than one X11 server running. (ie: my desktop is on ":1" but there is also a ":0" display)

Note: when using "auth=none", you can omit the "username:password" part. (and accept the risks that go with it)